Weird! The other thing I'd try is setting a max value to how many of a specific enemy unit can be spawned at once, but I can also think of several ways that might backfire. Either way, it's not too hard to just kite them and tag the exit, I even gave the game a pacifist run just to see how far I could get.
It's a very fairly-designed gameplay loop overall, great job on designing such a fun game ^_^