Hint: Lava lobster traps are not bugged, it's just that most lava pools don't have any life in them. There are more hints to the location in one of the fishing books.
Hint 2: Currently, the void lobster is the only lobster that can be caught with the lava lobster trap. The longer that a trap is in the lava, the greater chance to get a void lobster. Since there is only one creature the lava trap can catch in lava, if the bouy bobs... you caught one.
Hint 3: since the Void Lobster is a "Legendary" status, it's practically impossible to catch them if you are not lgm in fishing. Basically, they are intented to be the rarest lobster in the game (for now anyway) because you must have a trap that is only available randomly through the fishing quest, be in the right place, be lgm fishing, and get really lucky. The reward is an extremely rare lobster that can be made into a trophy.
General Hint: The way traps work is that they are on a pulse. Each pulse there is a chance to catch something. Each pulse, the chance to catch something Enchanted or Legendary goes up. Also, each pulse the chance to lose the trap goes up. After about 10 minutes, the chance to lose the trap is about 50/50. A trap can hold up to 5 lobsters or crabs (once it bobs 5 times, it's full).
BTW, we made the change you all requested and now lobsters and crabs from the fishing quest have the same reward weight as deep sea fish.