Iron Brigade is another example, sort of a steampunk/military action td game. Even though it's fairly different, Yet Another Zombie Defense HD might scratch the same itch and it's on sale for $0.51 right now.
I've only had this happen to me once on wave 24 of promenade NMHC. Otherwise, it's never been an issue but I haven't run glitterhelm yet. They eventually came out of the spawn door when it happened to me.