I can confirm this on 19.6.2 also, even through MPC-BE on HEVC content. The situation does seem to be resolved on 19.7.5. Funny thing is though, I don't recall there ever being much in the way of GPU activity during video decoding at all prior to W10 1903. Unless I'm mistaken? I thought a different part of the GPU was responsible for video decoding, not the main core.
RX480 W10 1903
Edit: It's specifically related to "GPU 0 - Video Decode" in task manager, not "GPU 0 - 3D", so they are separate.