There is no rule in 5e preventing damage stacking. I should be rolling 1d8(longbow)+1d8(CS)+1d6(HM)+Dex modifier, but I never do. If the lack of stacking is conscious game design there's no indication of it, though I assume it's a bug.
Also Colossus Slayer is completely inconsistent. It even applies on enemies with full HP most of the time.
