Well it's not that, I was actually there last night for a brief period and I got stuck because I couldn't find the ladder (hadn't been there before) and it wouldn't let me /home out of it...
~ @Bolthur
terrarium is set on a certain boundary of blocks, somewhat similar to how chunks operate in game
if you go too high in there you'll get the spawn speed boost back and access to commands, that's why you cant attack at the little ledge where you enter the arena but you can when you jump down
in my case I fell into void, in spawn you don't take void damage, but I had to wait a second to get out of the pvp boundary so I can run commands again
Hopefully that made sense
