Created attachment 146725 [details]
Screenshot of the issue
The battery plasmoid shows a large grey rectangle in its header, whichever theme is used. Removing the cache with "rm -r ~/.cache/*plasma*" fixes the issue (the leading * is necessary, otherwise the bug is still there), so this appears to be an issue with caching.
STEPS TO REPRODUCE
1. Click on the battery plasmoid
There is a large grey rectangle.
There is no large grey rectangle.
Linux: KDE neon
KDE Plasma Version: 5.24.0
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.3
This appears to be a corner case where the files didn't change on disk (which cas fixed in Bug 445516), but metrics are inherited from another theme incorrectly. We may want to invalidate the entire cache every time the Plasma theme is changed as well.