filtering new emails where? only the inbox is scanned for incoming mail to filter. increasingly I am seeing folk placing mail into things like "important" folders and expecting Thunderbird to "just find" their new mail. That is not how it works

I would assume NEW emails go to the inbox, because auto filtering does not work and therefore will not shift them to ANY other folder.

Clearly you did not understand or did not read my comments. I will assume that the concept of server filtering of mail into say an important folder is not something you have encountered before. https://support.google.com/mail/answer/186543?hl=en#:~:text=See%20your%20important%20emails%20Next%20to%20emails,as%20important%2C%20search%20Gmail%20for%20is:important%20. Gmail uses a term like priority inbox to describe it. https://support.google.com/mail/answer/18522?hl=en&co=GENIE.Platform%3DDesktop Outlook uses the term focused inbox https://support.microsoft.com/en-au/office/focused-inbox-for-outlook-f445ad7f-02f4-4294-a82e-71d8964e3978

I am assuming nothing because I am dealing with jack public and the level of incompetence routinely demonstrate by Jack is mind boggling.

Have you consulted the support document of filtering? https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters regarding what to expect.

My next question is how frequently are you having Thunderbird check mails. Set it to a minimum of 10 minutes and see if that changes anything. I have seen on more than one occasion that the second mail check commences before the first concludes resulting in odd incomplete behaviors.

What exactly is the filter you are having issues with? I find folk filtering in the body of an email on IMAP accounts really need to change the filter processing to be after junk classification to allow time for the actual message body to arrive locally to be filtered.

Seems a simple reboot of Thunderbird solved the problem, but thanks for your condescending comments.