Can confirm - have Radeon pro 5600M 8gb. It does it with Displayport and HDMI on my system. I'm guessing they'll not bother till MacOS Monterey - which has been my experience with EGPU and weird issues like this in the past when they're hard at work on a new OS. Not great - it's been going on for ages. Hardly what you'd expect when you pay the Apple premium for high end hardware