Hello, I'am having this problem (ArchLinux + Plasma5) that is mostly noted when I'm hovering with the cursor over long listed plasma application menus and panes, or scrolling the scrollbar, like the application launch widget, or system settings, which have many listed entries on them.
The text get 'missrendered' like the picture below:
https://imgur.com/wOOnxri/

I always noticed it happening on the desktop panel when the cursor is idle for some minutes, some other times the texts from the panel just 'vanishes':
https://imgur.com/DpJ7MWX/

and in applications that refreshes text periodically, like sysmonitor:
https://imgur.com/SIL9Iej/

Interestingly, it does not occur in sysguard application. Neither in any other application that are not KDE'S (like long listed folders in dolphin). The text gets correctly rendered and placed when the cursor or scrollbar gets idle for some seconds (2-3). I've verified ksyslog, nothing there. I have tried many configuration settings for fonts(dpi and anti-antialising settings), compositor and desktop effects, I've tried to install and reinstall both AMD drivers, xorg, qt and plasma. Actually, it started some weeks ago, I decided to make a fresh installation of Arch+KDE, for other reasons, and thought that this problem would go with it, it didn't. I currently have ttf-dejavu and ttf-liberation installed. The problem occurred again as soon as launched plasma after the fresh installation. I have a MSI R7 360 graphic card, and it is working very well. I have a secondary monitor, and it exhibits the same glitch.
I really searched for anything that could be related, it looks like a text rendering problem with kapplicatitons (qt maybe?) but I'm out of clues, I really would like to give you more information, but I really don't know what could be helpful, if somebody could give me ANY directions to correct this, I really like KDE, but this is not just a minor inconvenience, it really interferes with my experience with KDE environment.
Thanks in advance.

Which exact Plasma version do you use? I have never seen this here on my computer, neither the Laptop screen nor the External monitor. I wonder if it is not actually graphic-card specific...

Sorry, I forgot to state my system specs, here they are:
Versions from KDE system settings info:
Operating System: Arch Linux
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.82.0
Qt Version: 5.15.2
Kernel Version: 5.12.9-arch1-1
OS Type: 64-bit
Graphics Platform: X11
Processors: 12 × AMD Ryzen 5 1600 Six-Core Processor
Memory: 11.7 GiB of RAM
Graphics Processor: AMD Bonaire Series

And some update of my troubleshooting:
-The fonts do not get corrupted in the application launcher and panels (but remains in other kde applications like system settings) when I start the plasma session with wayland. I tried using xf86-input-evdev because it was similar to this bug: https://bugs.kde.org/show_bug.cgi?id=358472, but it didn't change anything.
- The issue does not seem to be within KWin, as I started a session using openbox as WM and the problem remained

I don't think that it is my graphic card, but could be something with AMD driver, because the issue is only presented in kde applications, I found a post on Arch Linux forum with the exact same issue:
https://bbs.archlinux.org/viewtopic.php?id=266674, but nobody answered it.

Hm, the main difference is you using Wayland while I still use the X Server, my graphic card is also AMD (see signature)

I used wayland only for testing porpouse, but I use X Server. I found out that this is a bug in the mesa driver, seems to affect AMD gpus, a workaround is described here:
https://gitlab.freedesktop.org/mesa/mesa/-/issues/4764
but I lack the technical knowledge to perform it. Downgrading the package doesn't help much either...

It doesn't affect all AMD GPUs, as it obviously works fine on mine...