You have to use a level 65 rare upgrade component, unfortunately. The back pieces have higher base stats than other exotics, though. If I calculated correctly, in the case of Berserker’s, there’s actually a +6 boost in the main stat (power in this case) and an even match on the secondary stats (precision and ferocity) with the level 65 upgrade over level 80 exotic back pieces using the level 80 upgrade. Just have to be careful next time. That being said, it did catch me off guard as well at first. But I don’t believe this is a bug.