For whatever reason when watching YouTube its the encode blocks of your GPU, not the Decode blocks that are active. Hardware acceleration is working, just not in the way you think. YouTube uses VP9, not H.264/H.265 for their videos above 1080p.
Same issue here on any Adrenaline 2020 versions, Windows 10 1903. 50-70% GPU encode when watching 1440p Youtube video, and browser (tested on Chrome, Firefox and Edge) stutters. Adenaline 2019 would have 0% encode and 10-20% decode.