Thats a bug then though.
Original D&D5 rules clearly state it shouldnt stack. Only way to get legally two extra attacks is being Fighter 11. And for three attacks you need Fighter 17.
It is not a bug.
It hasn't been changed for 4 patches while being "reported" since the release. On top of that, it is coded to stack and not like any other extra attack.
Too powerfull or not is another matter... But I'm personnaly glad it stack ! It allow many interresting melee build.
