Thanks for reporting this. I looked into it and tracked down the issue to the following.
I have submitted an MR which will hopefully be merged soon.
I believe you compile from code, can you make the following change and test?
(I'd have you just use the latest code but I don't necessarily recommend you using that right now.)
In the file kstars/kstars/skycomponents/artificialhorizoncomponent.cpp
find the function called appendGreatCirclePoints (starts on line 268)
and comment out these 4 lines which are not far from the top of the function (lines 274-277) invent.kde.org/education/kstars/-/blob/m...oncomponent.cpp#L274
It is not perfect--it makes little jagged lines near the horizon (which is what those 4 lines fix in horizon mode and were supposed to fix in equatorial mode too), but that is far better than the current rendering mess. I need some consulting to get it totally right, if you have any clue how to fix, let me know!