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. )

I use Calender's feature of repeating events to pay and track upcoming household bills. I have setup repeating events on days that household bills are due. Once paid, I edit the single repeating event as "Edit only this occurrence". I will change the title and sometimes the description box. I then Save and Close. With the upgrade to Thunderbird, most of the edits will be lost and the original repeating events reappears. In past versions of Thunderbird, the edited events would be saved.