With IMAP, messages are still accessible in the desktop.

Thanks, but with IMAP, my emails are not in my desktop storage. I have trust issues with Google. If not, I would just log in to mail.google.com from a browser to look at them and wouldn't need a desktop email client like Thunderbird.

I fail to see the logic in your changes - you write that you don't trust google and choose pop for that reason, but then choose to use the option "Leave messages on server".

That said I see only one bug report that might match your issue, and it is quite old https://bugzilla.mozilla.org/show_bug.cgi?id=821587.

I log in to mail.google.com to purge messages from the server rather than have the desktop email client do it. Thanks for pointing out the bug report. I don't think that's my problem because it's occurring with new messages, but it's enough to tell me it's potentially buggy. I'd try a Thunderbird reinstall but the import process from my converted outlook PST file was ugly and I don't want to repeat it. I guess I'll be sticking with Outlook, (despite also having Microsoft trust issues). Thanks for your help.