I have a Vega Frontier Edition, running stock speeds and voltages. I had it attached to an LG 1920x1080 display panel via HDMI and everything was great. When I recently attempted to connect it to my LG 55SJ8500, which supports 4K HDR, I started having strange issues with loss of HDMI signal. Just for sanity's sake, I DDU'd from safe mode and reinstalled drivers, and the issue persists.
It does appear that the issue is less frequent while using the Pro drivers vs. the gaming drivers, but I still find the problem to be unacceptable. Most of the time I'll get a horizontal bar running across the display, about 30-50 pixels tall, comprised of seemingly random-colored pixels. This seems to happen shortly after booting into Windows, and a lot of times it can be triggered by bringing up the Radeon/RadeonPro settings window. This bar lasts about 100ms or so, and doesn't follow any noticeable pattern as to where on the screen it appears, top vs. bottom.
Other times, the display will go black, but audio from the content source or game will continue as if nothing were wrong. Indeed, if video capture software (OBS) is running, it records the images fine, it just appears to be a dropout via the HDMI port. If I toggle the display's HDR setting, sometimes it restores the image. Other times, I have to change HDMI inputs on the display before it properly returns the image. The issue definitely seems to be tied to HDR, but the problem is that my display will enable HDR automagically if it detects HDR from the source so I can't simply force HDR "off" to test it. I've disabled HDR in windows and have still seen the display intermittently attempt to switch to HDR while running some full-screen applications, so it appears that a spurious signal is being sent which is triggering an uncommanded switch to HDR.