OK, tentative breakthrough. After all this I think this has been caused by my HDMI expansion card being installed in one of the unsupported ports.
Hey, that sounds new to me. Is there a (definitive) link that describes supported and unsupported ports for the HDMI plugin?
Dunno where this is definitively documented but it’s the 2 slots closest to the back.
Port 2 is the only one that doesn’t support display out. If you guys have TB3/4 docks and they have unexpected behaviour you can plug it into port 4, and force an USB3.2 connection instead of TB which is often more stable, however if your hub doesn’t have a decent MST chip and you are trying to run multiple high resolution displays out of a single port, you are going to have a bad time.
Also the back ports are the only 2 out of the 4 that are able to run TB3/4 albeit if you run Linux its not the best experience, but that has been pointed out multiple times to be an AMD issue, TB is more stable on Windows than Linux according to some users in the forums.