I'm having a similar problem. I have monthly, recurring calendar events. Every month I edit that specific month's event with additional information - not the whole series. Since 1/13/21 all the the past specific event edits have disappeared. The basic recurring event remains intact.

( Probably same problem ) When Thunderbird started this morning, I got 9 reminders, most for instances of tasks that had been marked completed. Most instances of all recurring events for at least the last year had been "unmarked". I marked the instances for recently completed tasks. Shut down and re-started Thunderbird. Tasks unmarked. Repeat, - arg ! Installed 78.4.3. Some of the completed marks re-appeared. Seems to be the same repeated tasks. One of which was marked completed for at least 3 months ahead. ( Presumably all instances. ) So 78.6.1 unmarked repeated tasks on startup, not just on showing, but in the database. Database seems to be working correctly now. Of course that leaves a hash on past events, but I think that that is ignoreable. I have about 13 recurring tasks per month, going back several years. ( Deleting the past tasks is only a pain. Past events would be worse as that's records. )