diff --git a/packages/relay-test-utils/__tests__/RelayMockPayloadGenerator-test.js b/packages/relay-test-utils/__tests__/RelayMockPayloadGenerator-test.js
index 913c0d147e16..56699f4fe8e1 100644
--- a/packages/relay-test-utils/__tests__/RelayMockPayloadGenerator-test.js
+++ b/packages/relay-test-utils/__tests__/RelayMockPayloadGenerator-test.js
@@ -2201,3 +2201,210 @@ describe("Aliased linked fields without arguments don't overwrite each other", (
     );
   });
 });
+
+describe('Plural linked field selected on both a concrete selection and an abstract InlineFragment at the same level (#5258)', () => {
+  // Regression test for facebook/relay#5258 — currently demonstrates buggy
+  // behavior; flip assertion when the underlying bug is fixed.
+  //
+  // Scenario from the issue: a query field returns a concrete type that
+  // implements an interface, and the same plural linked field is selected
+  // both directly (concrete path, returning the concrete element type) and
+  // via a fragment spread on the interface (abstract path, returning the
+  // abstract element type). Because the field's return type differs across
+  // the two paths, the Relay compiler does NOT dedupe the selections — the
+  // normalization AST keeps the concrete `LinkedField` plus an abstract
+  // `InlineFragment` that carries its own copy of the same `LinkedField`.
+  //
+  // RelayMockPayloadGenerator's `_traverseSelections` processes both
+  // selections sequentially. On the second pass, `_mockLink` reads the
+  // already-generated array out of `data[applicationName]` and passes the
+  // whole array as `prevData` to each new element generated by
+  // `generateMockList`. The result is `[[e1,e2,e3], [e1,e2,e3], ...]`
+  // instead of `[e1, e2, e3]`.
+  //
+  // Scope: this test only covers the array-shape symptom. The related
+  // `mockData[TYPENAME_KEY] = selection.type` symptom from the issue's
+  // "Root Cause Analysis" (point #3 — abstract type forced onto already-
+  // generated concrete data) is a separate symptom and is not asserted
+  // here.
+  //
+  // The OSS test schema does not naturally model a concrete-vs-abstract
+  // return-type divergence on the same field name (every plural field
+  // returning an interface returns the same `[Actor]` from both the
+  // interface and its implementers, so the compiler dedupes them — a
+  // schema-extension prototype with a divergent `[ConcreteEdge]` vs
+  // `[AbstractEdge]` interface implementation was tried and the compiler
+  // still folded the InlineFragment because the parent field's static
+  // type pinned the abstract narrowing). To mirror the issue's
+  // "Standalone Reproduction Script" exactly, this test hand-constructs
+  // a `ConcreteRequest` matching the AST shape from the issue body.
+  // `MockPayloadGenerator.generate` walks the AST and does not consult a
+  // schema, so the type names below need not exist in the test schema.
+  test('BUG: produces nested-array shape instead of a flat array (#5258)', () => {
+    const request: $FlowFixMe = {
+      kind: 'Request',
+      fragment: {
+        kind: 'Fragment',
+        name: 'RelayMockPayloadGeneratorTest5258Query',
+        type: 'Query',
+        metadata: null,
+        argumentDefinitions: [],
+        selections: [
+          {
+            kind: 'LinkedField',
+            alias: null,
+            name: 'items',
+            storageKey: null,
+            args: null,
+            concreteType: 'ConcreteConnection',
+            plural: false,
+            selections: [
+              {
+                kind: 'LinkedField',
+                alias: null,
+                name: 'edges',
+                storageKey: null,
+                args: null,
+                concreteType: 'ConcreteEdge',
+                plural: true,
+                selections: [
+                  {
+                    kind: 'ScalarField',
+                    alias: null,
+                    name: 'cursor',
+                    args: null,
+                    storageKey: null,
+                  },
+                ],
+              },
+            ],
+          },
+        ],
+      },
+      operation: {
+        kind: 'Operation',
+        name: 'RelayMockPayloadGeneratorTest5258Query',
+        argumentDefinitions: [],
+        selections: [
+          {
+            kind: 'LinkedField',
+            alias: null,
+            name: 'items',
+            storageKey: null,
+            args: null,
+            concreteType: 'ConcreteConnection',
+            plural: false,
+            selections: [
+              {
+                kind: 'LinkedField',
+                alias: null,
+                name: 'edges',
+                storageKey: null,
+                args: null,
+                concreteType: 'ConcreteEdge',
+                plural: true,
+                selections: [
+                  {
+                    kind: 'ScalarField',
+                    alias: null,
+                    name: '__typename',
+                    args: null,
+                    storageKey: null,
+                  },
+                  {
+                    kind: 'ScalarField',
+                    alias: null,
+                    name: 'cursor',
+                    args: null,
+                    storageKey: null,
+                  },
+                ],
+              },
+              {
+                kind: 'InlineFragment',
+                type: 'AbstractConnection',
+                abstractKey: '__isAbstractConnection',
+                selections: [
+                  {
+                    kind: 'LinkedField',
+                    alias: null,
+                    name: 'edges',
+                    storageKey: null,
+                    args: null,
+                    concreteType: null,
+                    plural: true,
+                    selections: [
+                      {
+                        kind: 'ScalarField',
+                        alias: null,
+                        name: '__typename',
+                        args: null,
+                        storageKey: null,
+                      },
+                      {
+                        kind: 'ScalarField',
+                        alias: null,
+                        name: 'cursor',
+                        args: null,
+                        storageKey: null,
+                      },
+                    ],
+                  },
+                ],
+              },
+            ],
+          },
+        ],
+      },
+      params: {
+        cacheID: 'relay-mock-payload-generator-test-5258',
+        id: null,
+        metadata: {},
+        name: 'RelayMockPayloadGeneratorTest5258Query',
+        operationKind: 'query',
+        text: null,
+      },
+    };
+    const operation = createOperationDescriptor(request, {});
+    const payload = RelayMockPayloadGenerator.generate(operation, {
+      ConcreteConnection: () => ({
+        edges: [
+          {__typename: 'ConcreteEdge', cursor: 'a'},
+          {__typename: 'ConcreteEdge', cursor: 'b'},
+          {__typename: 'ConcreteEdge', cursor: 'c'},
+        ],
+      }),
+    });
+    // $FlowFixMe[unclear-type] payload.data is intentionally untyped here
+    const edges = (payload.data: any).items.edges;
+    expect(Array.isArray(edges)).toBe(true);
+    expect(edges).toHaveLength(3);
+    // BUG: each entry is the entire previous edges array (with extra
+    // `__typename`/`cursor` properties bolted onto the Array instance
+    // from the abstract InlineFragment pass) instead of a single edge
+    // object. The exact buggy shape is asserted with a strict structural
+    // match so any "improvement" (nested arrays collapsing partially,
+    // elements becoming single edges wrapped in an array, missing
+    // properties, etc.) breaks this test loudly. When the bug is fixed,
+    // replace the three deep assertions with the flat shape:
+    //   expect(edges[0]).toEqual({__typename: 'ConcreteEdge', cursor: 'a'});
+    //   expect(edges[1]).toEqual({__typename: 'ConcreteEdge', cursor: 'b'});
+    //   expect(edges[2]).toEqual({__typename: 'ConcreteEdge', cursor: 'c'});
+    const expectedBuggyEdges = [
+      {__typename: 'ConcreteEdge', cursor: 'a'},
+      {__typename: 'ConcreteEdge', cursor: 'b'},
+      {__typename: 'ConcreteEdge', cursor: 'c'},
+    ];
+    for (let i = 0; i < 3; i++) {
+      expect(Array.isArray(edges[i])).toBe(true);
+      expect(edges[i]).toHaveLength(3);
+      // The whole edges array gets repeated as each element.
+      expect(Array.from(edges[i])).toEqual(expectedBuggyEdges);
+      // Buggy second-pass also bolts the abstract path's own scalar
+      // fields onto the Array instance (taking the first generated
+      // edge's values).
+      expect(edges[i].__typename).toBe('ConcreteEdge');
+      expect(edges[i].cursor).toBe('a');
+    }
+  });
+});