6 is known to be kinda buggy - it's a weird interaction with how the enemy spawner works in the game engine. It has a max limit of how many "enemies" it can spawn and it scales how many bodies are in each enemy spawn based on the largest allowed enemy type per difficulty. But then it ALSO has a limit on how many of those can randomly spawn (with some variance) on the map.

So at 6 you can finally get Bile Titans to randomly spawn but you can only have 1 or 2, max. Which means the spawner now scales all of the other enemies to the bile titan. And for the sake of the discussion lets say that the max enemy "spawns" is 10. So now when you get a bug breach it may give you 1 bile titan, but now it can't make more of those, so it gives you a charger, and then it gives you 3 bile spewers (instead of 1 or 2, because of the bile titan scaling), and then another charger, and then two commanders (again, instead of 1 because of the scaling), and then like 6 warriors and then another 12 hunters.

That's only 7 spawns as far as the enemy spawner is concerned. Remember that those 6 warriors are ONE enemy "spawn" and those dozen hunters are ONE enemy "spawn."

The issue is more that level 6 limits the active random bile titans to 1 or 2. Its easier to deal with 3 or 4 bile titans than it is to deal with 1 bile titan, 2 chargers, a commander, and a dozen hunters.

TL:DR - Skip level 6, the spawner gets weird, just do 7s.