I was on an early game crimson world and decided it was time for the brain of cthulhu. I had noticed that the Flaming Mace I had been using recently basically creates a shield of damage and figured it would be good for the first phase. I crafted a bloody spine and spawned the brain. All seemed to be going well until I finished off the last creeper at that moment the brain teleported away and a few seconds later despawned. I tried again with the same result. I tried a third time with a different weapon and the fight went as it's supposed to.