Skaijie,
I didn't notice is occurring whilst swimming, but I couldn't test with elytra since I couldn't take off due to the input lag.
OcelotOnesie, no, the bug that you are suggesting that overlaps mine does not. The bug that I encountered does not include shifting hitboxes, nor does it require an elytra to cause. This causes simply by walking into a block at face level without a block being present at foot level.
Seeing how Mojang lumped my report with this, and seeing how their solution is to update drivers, I'm wanting to confirm that I have the latest drives from Intel and Nvidia, and the problem still exists. This isn't just a problem with drivers. Both graphics cards worked to my expectations prior to this update, and still show acceptable work in all other tasks that I throw at them.
GPU(s):
Intel UHD Graphics 630
Nvidia Quadro P1000
Update: I'm also noticing that some are guessing vsync. I don't have vsync active on my card, and am still having the problems listed by others.
After the 1.16.201 Hotfix, I noticed that the input lag has gone down, but is still present. It's now somewhat playable, but not a great experience.