MobStages-1.12.2-2.0.8

Currently using MobStages on a server and it works perfectly aside from one thing.

I've left ranges at default, so from your CF page it should be 256 but end-game gated mobs are still spawning around my base even when I'm 2k+ blocks away from it.

I'm using FTB for chunk claiming/chunk loading, so it might be that it's counting as 'me' and continuing to spawn gated mobs around my base as if I was there?

Main problem is I've got end-game mobs unlocked while other players aren't as far along and don't have the gear to survive them.

Any advice on this Darkhax?

It sounds like the mobs spawned in those chunks while you were there and have not de-spawned yet, probably because the chunks were kept loaded by chunk loading or the mob in question has properties which prevent despawning. This is mainly a technical limitation of intentional vanilla/modded mechanics.