Sounds like Ubuntu underneath your Plasma. I’ve had the exact same experience when using Neon, Kubuntu, and Ubuntu+KDE. I install any non-Ubuntu based distro with KDE (like openSUSE) and whiz bang everything is working again.
The last time I tried it it crashed just from moving the panel around on the desktop. After a reboot it didn’t do it again. Plasma just does odd things like that sometimes.
And if you used Plasma 4 all the way up to around Plasma 5.15/5.16, Plasma was practically unusable due to instability. It’s why Plasma stopped being the default DE of choice and Gnome took over.
Plasma has improved a lot over the past year or two in particular, but it’s not close to as stable as, say, Gnome or Cinnamon.