https://bugzilla.kernel.org/show_bug.cgi?id=200695
--- Comment #37 from Adam (magicmyth@magicmyth.com) --- As several comments mention different behavior between the different connector types I found a spare DVI cable and tried connecting to the same monitor via that input and it worked! So once logged in I plugged in the HDMI cable and switched input and it worked just fine (monitor was glitchy at first but once I disabled DVI output in monitor configuration it worked perfectly). To confirm this was indeed working with the DC code path I tested audio over HDMI and it worked just fine (and does not work with DC disabled, as expected). So I rebooted with both the DVI and HDMI in and the output over HDMI worked all through boot. I then disconnected the DVI and rebooted and the HDMI continues to work so far. Unfortunately I don't have a displayport monitor around to test if that behaves as weirdly. Just to be clear, I had tried to make this monitor work for a couple weeks over HDMI and the only thing that worked was amdgpu.dc=0 until I plugged in the DVI connector so this was not a one off fail.
I forgot to mention before that when the HDMI output was not working with DC my Xorg logs also showed no monitor detected.
I can attach the two different Xorg logs if that helps?