No idea how to better describe the issue than this. It seems to happen on some apps more than others. The two screenshot attached is from Konsole but kwrite and kate are the worst offenders. I’m using wayland.
KDE Plasma version 6.4.1
KDE Frameworks version 6.15
Qt version 6.9.1
linux kernel version 6.15.4
Arch
After some more testing and digging on the web, my issue sounds identical to this bug 373232 – Horizontal lines with fractional HiDPI scaling
It’s quite an old bug though so not sure whether it reappeared or if what I’m experiencing is caused by something else, but that’s exactly the same symptom. I’m not seeing any real solution in that thread in any case, only mentions that some version of plasma 5 fixes the issue, which clearly isn’t the case. So I’m not sure what I’m missing 
Here’s another screenshot of how it looks:
Setting the display scaling to 100% “fixes” it though I would really like to keep my scaling of 115% as I find this way more comfortable. If that helps, I’m using a QHD (2560x1440) screen.
No more lines after switching to 120% scaling in Quad HD, but it’s still a workaround. Can notice lines even when moving the mouse rapidly up and down, cursor leaves lines similar to your screenshot, 120% scaling eliminated the issue, so it can be used as a temporary fix.
Yes, I have also settled on the same 120% setting and haven’t noticed the issue anymore. I 100% agree with you though in that this is not a fix but a workaround. Personally it’s one I can live with but still.
I don’t know what the best course of action for properly reporting this would be, between creating a new bug in kde’s bug tracker or adding to the existing one I linked above, knowing that that one’s last activity was 2 years ago and seems to be more focused on Konsole
