The top tag is 'Important'. Are some messages being marked as such by your mail provider before they are downloaded to TB? Check the account through webmail.

Thank you for the suggestion, but, no, 'Important' is not the top tag in my list. For the record it's actually 'Training', but the emails being randomly tagged with this include ones from Amazon, and others with no relevance to training.

If you have labeled number one as training, all good. But all the information you have been provided by SFHowes is just as relevant regardless. 1 is normally considered important and most IMAP account synchronize tags (or the first 9). With the trend to these priority inboxes (with various names) many providers are marking mail as important (entry 1 in the list) to facilitate their inbox segregation. (Include Hotmail/outlook and google in that many)

Okay, that's helpful information, but unfortunately doesn't solve the issue; which is that this only started happening within the last month - suddenly 'random' emails are being tagged. Why not earlier?

Secondly, my mail is to a personal domain, not a big provider, and I have done nothing (so far as I know) to change any settings that might trigger such sorting, e.g. filters.

So, I am wondering if this is some kind of bug, and if anyone else has a similar experience.

Someone reported this issue in a bug recently and it was discovered that this was not Thunderbird. It seems to be nextcloud mail issue and you need to turn off the Automatic message importance classification. NextCloud are aware they have a bug, so basically this is not a Thunderbird bug. Read info here: https://bugzilla.mozilla.org/show_bug.cgi?id=1718853

Thank you, that's interesting. I am running Nextcloud - on a separate server on the LAN, but don't typically use the mail app although it is enabled.

I'll have a look and report back - it certainly seems like a likely prospect.

Okay, several days have now passed since switching off the auto-tag feature in Nextcloud's mail app, and that seems to have resolved the issue. Thank you to Toad-Hall, and everyone who had a stab at working out what was going on.

Cheers.