Mail Rules misbehaving

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

rwzpv wrote:

Mail fails to move incoming emails to the mailbox specified in Rules, then won't let me move the mail manually to the correct mailbox

Make some changes—Rules:

Change Add Rule settings in Mail on Mac - Apple Support

https://support.apple.com/guide/mail/change-add-rule-settings-cpmlprefrulesadd/mac

Filter emails in Mail on Mac - Apple Support

https://support.apple.com/guide/mail/filter-emails-mlhl1f6cf15a/mac

Use rules to manage emails you receive in Mail on Mac ...

https://support.apple.com/guide/mail/use-rules-to-manage-emails-you-receive-mlhlp1017/mac

rwzpv wrote:

Mail 15.0 (3693.20.0.1.32) doesn't follow my Rules, putting emails into completely unrelated mailboxes.

Does Mail have a limit on the number of mailboxes?

That would be hard to believe...

Call Customer Support  (800) MY–APPLE (800–692–7753)

or on line  https://getsupport.apple.com/