Few more random details:
1) monitor is attached via VGA, the only output on the motherboard
2) different monitors show the same behaviour
3) GDM vs SDDM, no difference
4) in both GDM and SDDM, the mouse cursor is drawn correctly
5) Wayland vs Xorg, no difference
6) with older kernels the system would freeze the moment KMS kicked in, so this
is already a big improvement