I'm not sure If I'm doing something wrong here but for some reason every time I restart my server I have to re-enable monster spawning (using /mvmset monsters true) despite it already being enabled in the config.

This is happening to me too and has been reported before I believe - a tip until it's fixed is that doing /mv reload works just as well as the set monsters command :)

i think the problem is 1.16 only

I have this too but only on one world = maybe the one I first spawn in

ah I forgot I'm also on 1.16.1 (PaperSpigot 138) also I looked into it a bit I activated debug and it throw errors but mainly on the new materials (Blackstone) . I haven't tried to make a portal out of it but I guess it wouldn't end well either

It's possible this is a Spigot bug, but I'll have to do some more digging to be sure.

If you have everything up to date you should fix the problem.

solved for me I don't know of it was multiverse the version I had was from feb or earlier but there was a newer built but without version increment . Edit I also updated the PaperSpigot version to 148

With Paper spigot 150 it seems broken again testing 158

I'm going to close this as a duplicate of #2318.