https://bugs.freedesktop.org/show_bug.cgi?id=108031
--- Comment #5 from Alexander Schlarb alexander-fd@xmine128.tk --- Using the latest Debian kernel (4.19.0-2, which apparently corresponds to Linux version 4.19.16, with 4.19.0-1 having corresponded to 4.19.13) this issue does not affect me anymore.
I looked through the kernel commits between the versions but none looked very related, there was some stuff about DP MST which maybe somehow relevant since the display is connected using eDP. Dunno.
Anyways, works for me now.