Followup: For no apparent reason, automatic filtering works again now, after having logged out and logged back in.
It is strange, because when the filters didn’t work, I closed KMail completely and then restarted it, but that didn’t make any difference; it still wouldn’t apply automatic filtering on incoming mail. So perhaps it’s got something to do with a memory-resident shared library.
That said however, it has already happened in the past that KMail would also not apply automatic filtering on startup ─ I’ve got it set to automatically check for mail when the program is started ─ but then that never persisted, and subsequent new emails would always be neatly redirected to their intended destination folders.
So the bug was already there, but for some reason, it chose to persist across the entire session ─ and even after a KMail restart ─ after the 2021.08.09 Stable Update.
And yes, of course, I always update my system from the command line while completely logged out of Plasma, and I also always reboot my system after the update. But somehow, the bug got triggered and persisted for two days, until I completely logged out of Plasma and logged back in.
This sort of behavior implies that it’s not just a straightforward bug, but possibly something ado with a race condition and/or a symbol lookup. And I’m not a programmer, but in my understanding, then that in turn would indeed be pointing at a shared KDE library, because simply restarting the application wasn’t enough to get rid of the bug ─ it needed a complete logout.
Anyway, thanks to @Fabby, and anyone else who read through this thread and bumped the votes for this bug over at bugs.kde.org. 
