Example.
Wave 187. The wave contains a boss (one or more) with two phases. If player beat the 1st phase of boss, game starts the next wave, which should not happen, because the previous wave has not yet been cleared completely.
It’s not a bug. Bosses with two phases always count as being defeated when the 1st phase is defeated. It’s something that you have to plan for ahead of time.
^ This is the case.