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For now I can not re-produce your issue, also I do not use Lightroom so I have no idea about it.
But, for MPC-HC and youtube , this is my experience(on latest Creators drivers):
1.If your Windows 10 is post Creators, pleas use 17.6.2, use older pre Creators driver with DXVA will caused TDR on my case.
2.MPC-HC or DXVA player : try to switch to DXVA native if you are using copy-back.
3.Youtube: Chrome or Firefox has a plug-in "H264ify", try it. MS Edge is default h.264 so no need to do anything.
Give up VP9(4k) for now.(Chrome default is to use VP9)
4.For clean testing,while using DXVA, disable Relive feature on Radeon Settings. and not using Lightroom.
For the VP9 I have a thread but never got a real answer so only can give it up:
If, you still have issue on using H.264 DXVA native , I wonder maybe the card or other hardware things has some issue.
I use RX470 , I wonder RX480 is not so different from RX470.(but only performance)