Frontend Applies Default Border Width for border-${side} Without Explicit Width Property

[amunpote]

Description

Border values set using border-${side} without explicitly defining a width property are assigned a preset border width on the frontend, regardless of the border color settings. In the editor preview, this behaves correctly and no unintended border is applied. However, on the frontend, a default border width is rendered unexpectedly.

This behavior appears to be triggered by theme patterns and was working correctly prior to WordPress 6.9.4.

Expected Behavior:
If border-${side} is set without a width property, no preset border width should be applied unless explicitly defined.

Actual Behavior:
A default border width is applied on the frontend, causing unintended borders to appear, while the editor preview renders correctly.

Step-by-step reproduction instructions

1. Install and activate the FotaWP theme by CozyThemes.
2. Go to the Site Editor or Page Editor.
3. Open the Block Inserter and navigate to the Patterns tab.
4. Search for the FAQ Block pattern.
5. Insert the pattern into the page.

Save the page and view it on the frontend to observe the issue.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure

[talldan]

I think it's the same issue that was already reported in #70211, so I'll close this as a duplicate.

[amunpote]

@talldan, thank you for your time. I’ll make sure to thoroughly go through the repo before reporting any bug issues. Much appreciated!