There's currently an issue with how the classic end portal parallax tweak works. It'll end up turning white or glitchy and you can do this by spawning an Eldritch Guardian from Thaumcraft nearby or flying upwards and looking down at the portal to see it slowly change to white (this can occur much more easily when having a low render distance). This does not occur with the vanilla parallax and I've tested this without any other mods.

Any screenshots for this?

It seems like it‘s how the portal reacts to fog. I‘ll see if I can find a way around it.

Any screenshots for this?

Looks to be the fog like you said.

Did some digging and it‘s actually part of another long-lasting rendering issue: https://bugs.mojang.com/browse/MC-3607
Hooray! I guess.

True nostalgia backports bugs 😎