ZDoom

Moving to "on hold" since fixing this will be extremely difficult.

Because a portal consists of two scenes - the one in front of and the one behind the portal. And due to how movement interpolation is implemented it is very hard to determine if the player sprite should be drawn or not - because it may just be a remote view into the actual place where the player is.

Why does it even render the player sprite with chasecam off?

I've seen this happen in OpenGL too, by the way... a combination of bad framerate and bad internet connection during multiplayer. It's like the predicted camera position and the player sprite isn't matching up.

gzdoom-x64-g3.0pre-202-gbb1e927
It seems like when my framerate goes below 20ish fps, my player sprite becomes viable and quite intrusive.
(I realize you're not supposed to run BOA in software mode, but its the only mod I have that can get my framerate that low.)