I am surprised it works at all. Mystcraft can cause fatal crashes with SpongeForge.
mystcraft-1.12.2-0.13.7.06.jar in there. Check out our mod compatibility page on the SpongeDocs for more info.
I dont see anything for solutions for ae2 thats what the issue is isnt it? But yeah I see what you are saying weird how it is working with mystcraft
Update to the LATEST (rather than recommended) build of SpongeForge and see if that fixes it, a bug was introduced in 7.2.3.
Scroll down to “All builds” and the latest is the top one