I use Thunderbird on an Ubuntu workstation. Recently I upgraded to the developer version of 21.04. I can't prove this caused the problem but the calendar no longer displays right in Thunderbird, version 78.10.0.
I have attached a screen shot of this week's calendar. There are 9 events on this page but none show up properly.
I'm not at all sure what is broken here but this don't work in this config.