Boss Venom doubling up on Invisibility on a single tile

Pogo

Venom had several Invisibility tiles on the board. I destroyed one, and the resulting cascade caused another to lower a few rows. On Venom's next turn, he laid his new Invisibility down on that existing Invisibility tile that had just been lowered.

Not sure how he tracks the location of his tiles from turn to turn.

jamesh

Is it possible that there weren't any free purple tiles on the board?  If that was the case, then Venom wouldn't have been able to create any new invisibility tiles on his turn, effectively doing nothing.