I never had any love for SDDM, and even less since it followed GDM off of tty7. I run mc on tty2. So, I no longer have any 15.4/Plasma installations using SDDM, and can’t try to reproduce, and neither on 15.5. Instead, I use any of the same DMs as I use with TDE and KDE3: XDM, TDM or KDM3. If you have any interest in having the old behavior back, you might wish to make a switch, as all three still run on tty7 (I think - I’m not entirely sure about XDM).
Note you only have a very few weeks of support left for 15.4, so you might want to go ahead and upgrade to 15.5.
It’s possible that you could change the behavior (eliminate corruption) with a display driver switch. You have two competent choices: 1-radeon; 2-modesetting. Try switching. This elaborates.
Something else to check with your GPU is to make sure it’s using crocus dri rather than i965. Check it using xdriinfo. Which dri is in use is also part of inxi -Ga output.