I have searched the issues for this and I'm surprised this hasn't been reported before. In vanilla, when you die in the Nether and the End, you respawn in the overworld if you have no spawn point set. By default, with multiverse-core installed, you always respawn in the dimension you died in.
Of course, you can fix this by editing the respawnWorld setting in the config, this issue is purely concerning the default value not matching vanilla and confusing server owners who did not intend respawning in the Nether to be allowed.

This will be addressed by Multiverse/Multiverse-NetherPortals#155.

Oop well i found the same issue well i guess i wasn't stupid after all :/

Yea, the temporary fix is to run /mvm set respawnWorld <overworldname> <netherworldname> and /mvm set respawnWorld <overworldname> <endworldname>