I have a server called Puchacraft.
This has the version 1.16.5-forge-36.2.34

The issue happens when players teleports to a new location near to me, or usually when they come from the back of my camera.
They are completely invisible and I am only able to see them when they sit in a chair.

This happens with other entities, for example the mod: camera-1.16.5-1.0.14

Thank you for all your efforts.

If you need I can give you videos or pictures, just comment.

I mean, the camera is entirely unused and teleporting is not handled any different then normal walking. Try binding the debug key and check if that makes them visible again, but from the description it doesn't sound like this mod.

Closing as inactive.