Inside the nether portal just completely disappears, it seems like its when you get too far away, almost like the portal despawns. Look just looks like an unlit portal.

This is on forge server running these mods, we had a creative server for testing and had no problems like this, then as soon as we went to survival we started getting this problem.
List of mods:
(yes i know its a lot)

aether-1.12.2-v1.5.3.1.jar
animalnet-v1.12.2+b3.jar
AoA3-3.3.1.jar
betterportals-0.3.7.7.jar
buildcraft-all-7.99.24.8.jar
CodeChickenLib-1.12.2-3.2.3.358-universal.jar
Commons0815-1.12.2-1.4.0.jar
Computronics-1.12.2-1.6.6.jar
EnderStorage-1.12.2-2.4.6.137-universal.jar
energyconverters_1.12.2-1.3.7.30.jar
Forgelin-1.8.3.jar
ForgeMixinFix-1.0.0.jar
Galacticraft-Mod-1.12.2.jar
jei_1.12.2-4.16.1.301.jar
MicdoodleCore-1.12.2.jar
MobGrindingUtils-0.3.13.jar
mtqfix-1.12.2-1.1.0.jar
mystcraft-1.12.2-0.13.7.06.jar
NoMoreRecipeConflict-0.13(1.12.2).jar
OpenComputers-MC1.12.2-1.7.5.192.jar
OpenGlasses-MC1.12.2-2.2-50.jar
twilightforest-1.12.2-3.11.1021-universal.jar

Have the same issue, would be great to get a command to refresh portals
(edit )after disabling MobGrindingUtils the end portal reappeared.