SUMMARY
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I'm using KMail with Gmail using IMAP (details below) and every time I delete a message, run a filter that deletes messages, or expire old messages (using permanent deletion), after synchronizing with the server, the deleted messages re-appear in the folders they were previously in. Additionally, I checked the GMail web portal and the messages I delete in KMail are still there as well.

STEPS TO REPRODUCE
1. Configure KMail with an IMAP account (mine is GMail), using PLAIN authentication over Port 993 and SSL/TLS encryption. I'm also using App passwords with GMail if that's important.
2. Delete a message, or apply a filter that deletes that message
3. "Check Mail"/Synchronize account with IMAP server

OBSERVED RESULT
Deleted Message re-appears in mail folder.

EXPECTED RESULT
Deleted message remains deleted. Does not reappear in mail folder.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 5.16.2-1-MANJARO/plasmashell 5.23.5
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION