There's a pretty weird bug with the calamity biome in multiplayer:
Biome item right clicked in inventory, game freezes for some seconds. This is the result.
You can see the biome generated, but however this isn't true. They are all temporary ghost blocks. You can destroy them but nothing will drop. Lava is floating, and mobs are bugging and are able to go through the blocks the biome created (see the firebat).
After the server restarted, at the same place:
So nothing has generated at all. I don't know if this can be fixed but you can try