Quick summary

When using the Classic Editor to edit posts, the selected categories under Most Used tab are not saving.
This issue only occurs on Simple sites. It works fine on Atomic sites with Classic Editor plugin enabled.

Steps to reproduce

1. On a simple site, create some categories
2. Apply a few of those categories on posts so you can have a list of "most used" categories.
3. Open one of the posts using the Classic editor
4. Under Categories, click Most Used tab
5. Select some categories
6. Save draft or Publish the post
7. Check if the most used categories are saved

What you expected to happen

The selected categories under the Most Used tab are saved after the post is saved as a draft or published.

What actually happened

The selected categories under the Most Used tab are not saved after the post is saved as a draft or published.

Screenshots:

I selected a couple of categories under the Most Used tab:

After saving the post, the selected categories are unchecked.

Impact

Some (< 50%)

Available workarounds?

Yes, easy to implement

If the above answer is "Yes...", outline the workaround.

The workaround is to select the categories from the All Categories tab, which can be challenging for a site with a lot of categories.

Platform (Simple and/or Atomic)

Simple

Logs or notes

User report: 8833769-zd-a8c

Reproducible on Twenty Twenty-Four and Overlaid theme, on Chrome browser.