Been playing in the new direwolf20 1.16 mod pack was setting up a void base on a multiplayer server and the mulitblock doesn't form there, but does in the overworld.
Confirmed it does not work in single player either.

Void dim mod: https://www.curseforge.com/minecraft/mc-mods/just-another-void-dimension

srceen shots:

have checked with multipul teirs and all do the same.

I'm having a similar issue, however, even in the overworld, it won't form nor recognize it as completed

Edit: my issue was that I can't read and I made a mob controller with nothing in it

The only thing I can think off hand is that I need World Ticks in the dimension do to the multiblock forming.
There is mention on the issue page for that mod that "Time is frozen" which could match up with no world ticks.
There are a few other mods on their issue tracker not working with frozen time as well.

@TDderpy I'll need to download that mod and see what is happening. Not sure why a specific dimension would cause an issue but hopefully I can track down what the problem is.

@Ipsis sure, I'm currently playing on version 1.2 of the direwolf20 modpack. Will be updating the server to 1.3 shortly so will let you know if that changes anything.

Only thing I can think of that may cause it is that the dimensions doesn't follow default mob spawning rules. While it isn't a peaceful area (you can move mobs there or force spawn them) it doesn't have natural spawns, like a darkroom. So if your checking the game isn't in peaceful mode then maybe that's got something to do with it.