I have seen multiple people say this, and as far as I can tell every single one of them is wrong. If this is a real glitch, please provide some evidence (I'm not being rude, I just can't find any proof personally).
Also, trust me here, hunters mark and colossus slayer stacking would not break the game. From playing 5e for years now, it's not even close. There are much better sources of damage or stacking damage.
