Mail.app 14.0, macOS 11.2.3, IMAP email account.
I'm subscribed to several mailing lists and have Mail.app rules that sort mail from the list servers into specific mailboxes.
About one half of the list mail from the servers is correctly sorted, while a significant proportion isn't. If I select the unsorted mails in Mail.app's Inbox, then Mail > Message > Apply Rules, the unsorted messages immediately move to the correct specified mailbox. In all other respects, Mail.app runs correctly and quickly.
My setup uses Spam Sieve, but these Rules are below the Spam Sieve rule in Mail.app's Rules list, so SS has already done its exam and, I think, isn't involved in this oddity. The only Rule that runs an AppleScript is the Spam Sieve "Remote Training" Rule, which appears on the Rules list above the Spam Sieve Rule. The non-sort behavior also occurs if the machine (an iMac) is in Safe Mode.
The same issue has been asked about here and here. As suggested, I discarded all the rules, then quit Mail.app, restarted the Mac, and then recreated the rules from scratch. The same behavior continues.
Any idea what's going on here, or how I might improve Mail.app's performance?