I've run into this a few times. It's a pretty old bug and typically happens when Trio jumps before or at the same time as the crab does.
It's as if the crab is invisible and on the heads of all 3 pieces of Trio.
It does shake off if you can do the requisite damage to them though. The same as if they were actually crabbed. The problem is they're not actually crabbed so the damage has to come from enemies which aren't usually left alive.