I have no idea why, but they've said that they wont be fixing it.

arcanus mod below.
https://www.curseforge.com/minecraft/mc-mods/arcanus

Can you help in any way?

I don't know how that mod works, please explain what "Modern industrialisation causes arcanus mana to break" means.

It's a magic mod which impliments the magic like systems from wyncraft, The mana bar becomes empty or reverses for some reason when modern industrialisation is installed.

Have you tried this with ONLY MI, Arcanus and their dependencies?

indeed.

Believe me, i'm probably as confused as you are.

So, what was the issue?

It was a server side problem (even lan), i still haven't quite figured it out. I had to delete everything and restart.