Brand new RX 5700 XT, driver 19.7.2 (freshly reinstalled after DDU to fix the Direct3D issue, which is now fixed), CPU is a Ryzen 9 (3900X).
I first noticed a problem when doing some screen recording with the window-G in order to document some of the other issues I've been hitting (artifacts, see my other posts). The resulting .mp4 file couldn't be played with whatever standard player Win10 tries to use. I would get an error "Choose something else to play. This item is in a format we don't support" error code 0xc00d36b4.
Later I also noticed that iTunes install would complain that it cannot play video on this system and suggested I get an older version for "old GPUs". It indeed cannot play various videos I purchased.
Finally, digging a bit more, I notice Edge can't playback MP4's downloaded from sample video sites such as https://file-examples.com/wp-content/uploads/2017/04/file_example_MP4_640_3MG.mp4 (Chrome can though, so can VLC, I assume they use their own codecs).
It looks like something is wrong with HW accelerated decoding of mp4/H264 with the radeon drivers.