The cursor is effectively in one location until you release the mouse, so it inserts or deletes characters on a single row, which can lead to misalignment and then the Multi-Row Cursor mode stopping its effect entirely.
Demonstration of typing while selecting:
The cursor is effectively in one location until you release the mouse, so it inserts or deletes characters on a single row, which can lead to misalignment and then the Multi-Row Cursor mode stopping its effect entirely.
Demonstration of typing while selecting: