I have not tested this on every mob, but with skeletons, when the portal renders, it renders the head of the Skeleton in first person mode which covers the camera.

How to reproduce this bug?

Requiem seems to have many rendering issues with this mod.

To reproduce this bug, simply possess a mob/skeleton and be near a portal. My particular setup had a two-way portal for a tunnel that collapsed a large distance into a smaller one when moving through it. I.E. going around the portal was 8 blocks, going through was one.