https://bugs.freedesktop.org/show_bug.cgi?id=107390
--- Comment #16 from Justinas Narusevicius junaru@gmail.com --- (In reply to Harry Wentland from comment #15)
Can you try passing this on your kernel command line (with the bad commit): "video=HDMI-A-2:d"?
This will force HDMI-A-2 to report disconnected. I wonder if that helps.
It does! After forcefully disabling HDMI-A-2 everything looks to be working the same way as on pre ac916c914c3156e53505e9ea3a9d1495518bf873 kernels.