Yeah, I guess if it's not impacting a lot of people (which seems to be the case?), then my open-source comment is inappropriate.
I, too, have done some experimenting. I went back to 78.5.1 for the heck of it. I created a new recurring event and was able to delete or edit individual instances. But I could do that sporadically with the most-current update, too, so nothing earth-shattering, there. Nothing conclusive.
I was hopeful that regressing the version would also fix this other issue I alluded to: one of my calendars no longer syncing properly with it's associated Google calendar. Alas... the problem persisted post-regression.
Then I did some Googling about how to make the Thunderbird/Google sync work (I always forget the details), and came across a post saying I don't even need add-ons to do this. I've had both "TbSync" and Provider for "CalDAV & CardDAV" enabled as extensions! For whatever reason, I'd had to install both (I thought) to get the calendar sync to work.
I disabled the problem calendar in Thunderbird. Then I disabled both of those extensions and simply added a new calendar as a CalDAV calendar, syncing it to the Google calendar that had stopped syncing. This functionality is all right there in Thunderbird! And it worked!
So my current theory is that running those two extensions together were conflicting, somehow. And now I have a hypothesis: now that I have those two extensions disabled, my recurring series issues will not recur. Fingers and toes crossed.
This would explain why loads of folks aren't experiencing this "bug." It may also explain why the problem seemed especially bad with recurring events created before moving to TB 78. THAT'S when I added those two extensions. Just a thought.
But you, P.Mergey, are a test case, also. Do you have any such extensions running that you could try disabling to see if the "bug" goes away?