I'm testing out my modpack and went to the erebus to make sure nothing was broken.

I went back through the portal and was in the overworld for a few seconds before being teleported straight back to the erebus. This happens every time, even after breaking part of the portal blocks as well as the keystone.

Hello,
The Erebus portal will teleport you instantly with no cool down if you are in creative. I suspect that may be the cause if you are testing things out. If it's happening without a portal then, yeah, something is wrong in your pack, but I cannot duplicate this bug at all in any of my tests using just Erebus.
Cheers,
Me.

Is Stellar API in your pack? If so it could be related to this bug which is now fixed at their end.
MinecraftModDevelopmentMods/StellarAPI#10
If that is the case, please update and close this issue if it's the cause
Cheers,
Me.

Closing due to no reply and not enough info to go on.