I realize this thread is pretty old, but I had a very similar problem today of my enemy movement speeding up more and more each time after they were destroyed and respawned. I googled for a solution and came upon this page, but nothing above seemed to work.
I fixed my code by removing the movement function event listener (removeEventListener) in the destroy function. I guess the fact that the listener is never destroyed means that each time you create new enemies, they will have the previous listener(s) along with the one added each time the spawn/move function is called. This means the move function will have more and more listeners calling it, making each newly spawned character faster and faster.
I’m sure this won’t be helpful to the OP at this point, but maybe it’ll help out anyone else with a similar situation who comes across this page!