If a theme is loaded from a subdirectory, then its template parts are not properly loaded into the site editor. For example, if the theme is located here:

wp-content/themes/subfolder/themename

instead of:

wp-content/themes/themename

This has come up when folks test the TT1 Blocks theme: it used to work when you copied the entire theme-experiments folder into wp-content/themes, but it no longer does. This was fixed once before in #26391.

I believe this will be fixed by #27765, but opening an issue here to track the bug.