I ran into this bug as well and did some digging on it. Running the game vanilla, no-optional CDLCs, no mods, current stable (2.10.149954). It seems that the attack helicopters try to keep their nose at the horizon (vectorUp at, or as close to 1 as possible). It's a little less obvious with the base helicopters, but the Kajman is the most obvious (also fairly obvious on CSLA attack helicopters, where I started down this rabbit hole). I don't know if it's worth the effort to fix since it's a gain in altitude of like 50 m every 4 km of flying, but attached is an example mission of this.