The bug
Before release 1.3, while getting hurt there was a random chance of your characters arm to either jerk to the left or to the right along with the camera. Getting shot by an arrow or hit by a mob from the front, your arm and camera would jerk upwards. After 1.3, The camera and arm only jerk to the left no matter where you're hit or what you get hurt from.
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Can confirm. I have also confirmed that the camera moved correctly when messing around with beta 1.7 and release 1.0 a while ago.

Affects 1.16.1 and 20w29a.
Thank you for your report!
However, this issue is being tracked in MC-26678, which has been resolved as Won't fix.
(Btw, sorry for the late response!)
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