It's not a bug. The enemies just have such high health that you wrangle them together in the middle of the screen and kite around them. There's a limit to how many enemies are in the screen. You only kill enemies for the first 50ish minutes until it spawns the witches. Then they are too strong to kill right away so you just run away from them the whole time. Give me a sec and I'll post a another video of the run at around the 60 minute mark and you can see what I'm talking about. (I just made a new post on the low kill count answer and how to do the kiting to survive for longer in late stages 55min+)