The Tag called Important, is usually a tag that most servers also recognise.

So if server (webmail account) is applying an 'Important' tag to email then it's likely Thunderbird will recognise it and also display it. Logon to webmail acount and check to see if there are any Message Filters or other server settings which are applying 'tags' to emails.

If you have viewed emails via another device like a phone and applied an Important tag then server gets updated and thus an imap account will also display it. Or if someone else is also accessing that same Imap account then whatever they set will also get auto updated to server and hence to your imap account as well.

In Thunderbird: If you have created any 'Message Filters' then do any of those filters have an action to 'Tag Message' with 'Important' ? Check your Message Filters.

By any chance, could you be applying the 'Important' tag by accident ? If you select and email in list and press the number 1 key - this is a shortcut to applying the 'Important' tag to email.

Not sure if mean 'Tag' or whether you mean 'Important' folder. eg: Gmail imap accounts may display an 'Important' folder and gmail does have the ability to decide whether an email is 'Important' and thus can add a second label so same email can display in both Inbox and Important folder at same time. Fortunately, you can access webmail account and alter settings to stop gmail doing it.

Just a thought but do you have a Nextcloud server?

A few years ago I came across a similar case after someone posted a bug. It turned out that the bug was not in Thunderbird, but it was the server. At that time it seemed to be nextcloud mail issue and could be resolved if you turn off the Automatic message importance classification.

I did locate this link which seems nextcloud still has the ability to set email with Important tag. https://help.nextcloud.com/t/emails-are-tagged-as-important/177109

This link also mentions it's by design. https://nextcloud.com/blog/nextcloud-mail-introduces-machine-learning-for-priority-inbox/

Hey - Thanks for the reply. That has to be it (Nextcloud). I will test this out, but it does match as far as timing that it started about the time I started working with a Nextcloud installation and added that email account to check it out.

 Thank you very much. I don't know that I would have ever put that together. Will test. Thanks again.