Work fine on Apple Computer running Monterey, 1 Computer running Ventura and 1 computer running High Sierra
So fixing Apple Mail - can not duplicated this issue on any of these Machine
1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.
2 - Safe Mode will also eliminate Third Party Software, extensions and drivers from loading
3 - Does the issue present in this mode ?
4 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.
5 - If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.
6 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.
7 - Still have this computer issue - posted back a reply for additional suggestions.