I'm using kde plasma desktop on Ubuntu 20.04.
About once every 24 hours the bar that holds the launchers and clock crashes.

The bar itself remains visible. But the display doesn't update.

About once a day (and always every morning if I leave my machine running overnight) I notice that the clock in the bar is stuck.

When that happens, the graphics that represent minimised apps may or may not map to the actual apps.

What could be happening here?

Hi!

You could check the journal for errors of what is happening in the background (journalctl). But: Ubuntu 20.04 ships the end of life Qt5.12, so even if you'll find a bug or similar, it won't be fixed anymore. Even Qt5.15 is out of support for non-commercial users (but kept alive until migrations to Qt6.X are done), so the system is very old from Qts point of view.
As Qt is the main framework behind Plasma and other applications it would be better to install a new version of Ubuntu and check if those errors are already gone.