Hi,
I have 7 years old HP Elitebook laptop (i7 6 gen, 16GB RAM, 512SSD, Intel HD 520) on which I had KDE Neon for almost 4 years or so without any problems. 2 months ago I upgraded KDE Neon from 18.04 to 20.04. After this I noticed that apps (all apps, that means FireFox, Dolphin, VLC, Clementine player etc) after long running, randomly get GUI artifacts and doesn't respond on user interaction, but seems that it is only GUI issue because music player still playing and even stop when I click on stop button (but I don't see any hover animation etc). When I close it and run again then it is fine. It seems like some kind of X11 window refreshing problem but it also includes decoration (title bar), because close button (X) is also not hovering when mouse cursor is on it (but app is closed properly so accept click event).
I'm a guy who puts his laptop to sleep instead of power off and I noticed that this happens often frequently day by day when I put it to sleep. When I restart it then it is fine whole day. On previous KDE Neon version, I could put laptop to sleep over month without restarting it and never got this issue.
Even though this issue I noticed few months ago, I ignored it because I waited for brand new laptop and I thought that this is probably some weird issue on my very old KDE Neon installation and I probably messed up something with libs or config after all these years.
But now I have exactly the same issue on new Asus with i7 12gen, 16GB RAM, 1TB SSD and Intel Iris. This is fresh KDE Neon 5.25 installation from USB stick without any default changes in compositor settings etc. Same things happens after few days of putting laptop to sleep.

Extra info:
1. I work ALL time on external monitor. Even wake up from sleep is done by pressing Escape on external keyboard. On old laptop it was connected via Display Port and 144Hz refresh rate. On new one it is via HDMI cable and 122Hz (must buy HDMI 2.5 cable ) Can't confirm if it happens also on laptop's display because I work that way very rare
2. Seems that it happens much often to apps which have high refresh rate like VLC video player or scrolling web browser, but it also happened once or twice to Dolphin which was just minimized
3. After many days of putting laptop to sleep it is also starting affecting plasma components by itself, for example applications task bar, normally when terminal is doing some stuff, there is smooth progress bar animation, but when this bug occur, then even that animation is frozen and also mouse hover not highlight app on which cursor is
4. Don't know if such frozen GUI consume more CPU, I didn't think of it, I'll do if happen next time
5. Kernel 5.14.0-1051-oem. I had to use it because month ago generic kernel could not run Intel Iris drivers but recently checked that 5.15.0-46 generic can so I'll try to switch on it