If it was the bitrate surely UHD which uses higher bitrates works fine.
Just tested with iphone created videos, they all play fine with DXVA enabled for HD.
Here is one example.
DXVA RX480 1691hotfix for AMD Staff v2 - YouTube
if you check this video out UHD working fine with 24mbit/sec bitrate.
RADEON RX480 demo using DVBviewer 4k video - YouTube
this worked from day one.