Might be a mod conflict with out modpack, but I will have to do further testing, as there are 83 mods in total. I will report back later today with news. (The portal is right behind the wall to the left in that image)

(Yes, it's a conflict with better portals, as before it was installed this issue didn't exist)

Using Better Portals version 0.2.3

Is there any more news (preferably which mods are causing it) on this?
Can we at least get a log file and/or mod list?

I've not had time to test every mod for incompatibility, it's taking much longer than I expected. I can get you a mod list later today

Ah, It's Thaumcraft.

Tested with thaumcraft 6.1.BETA26 and BP 0.2.5

To replicate:
1: Create a world
2: Make sure there are entities of some kind on either side of the portal
4: Walk though the portal into the nether, and then back out into the overworld
3: Watch the f3 entity count go up about 1 per tick

Unfortunately I'm unable to reproduce the issue.
This is my setup:

Setup order: portal, overworld villager, go into nether, nether villager, go back to overworld, no issue
Then I restarted MC, went into the nether and back but still no issue.

Forge 2838
BP 0.2.5
TC 6.1.BETA26
Forgelin 1.8.3
Baubles 1.5.2

Hmm, Try adding the latest immersive railroading, I must've forgotten to uninstall it. it might be a combination of those two mods

Immersive railroading tracks also cause a lot of FPS drop when rendered next to a portal that otherwise doesn't usually happen. probably due to the number of elements in the model and the BP shaders. But that's less of an issue

Alright, I installed IR 1.6.1_1.12-1138 and TrackAPI 1.1_1.12 in addition to the mods listed above and now I'm able to reproduce the issue. Just entering the nether is already enough, I don't even have to leave it again.

That's super interesting, Because I tested with both IR and TC individually and neither had an issue. But combined they cause that bug...

Strange, I just removed TC and Baubles and it's still happening for me.
Also seems to happen when there are no additional entities in the overworld.

Hmm, I must've not walked though the portal while testing with IR. My bad