I made a mod in 1.7.10 to add rendering support for 1.8 skin format (by copying 1.8 code, sorry, I really want to use those skins).
In that process, I encountered this rendering issue, and I managed to fixed this issue.
If any of the mojang dev is interested in how I fixed this issue, here is the commit that fixed it: https://github.com/zlainsama/SkinPort/commit/d86b2d7670c0ab0fca98d474882583f286ec7a9f
What happened is the information used to render the player model isn't cleared after rendering, and it also does not get reset before rendering the first person hand.
Mod source repository: https://github.com/zlainsama/SkinPort
You need to enlarge the attachment to see it
It was working without any problem before today's update. (2021/06/30)