Try turning the threaded off and on again for the folder. It may be the user interface and the actual setting have become disconnected.

Sort-by threaded disabled, tbird re-started. Sort-by threaded enabled, problem persists. tbird re-started. Enabled/Disabled several times with no change. That Inbox seems impervious to threading, but only recently after upgrade to 102.7.0 (64-bit) on Linux.

I am struggle here, but try a right click properties and repair. just in case the index is wonky

The technical term "wonky" might be a bit of an understatement. After over 9 hours of repair, it's still grinding away ... see attached, please.

It got up to "1762 of 1701 in Inbox", followed by "24 of 24 in Inbox". I then repeated previous suggestions of un-threading, restarting the program, re-selecting threading, but the problem persists.

This is one of several imap accounts that I also use within googlemail on a phone, and I have to wonder if that app might have somehow damaged the index.

I just got en email from Dropbox. I replied but using my own address instead. The googlemail app on the phone threaded the reply properly, but Thunderbird doesn't.