Edit: This is now fixed. Apparently I had a PS4 exclusive bond equipped that dropped my level. PS4 to Xbox cross save success! ...That is until I log in and my power level is 656 with all 750 gear equipped. Powerful gear rewards drop at 750 even though the level shows 656, so I'm not sure if it's just an error in displaying the proper level. It even shows this lower level in the Destiny app. Just wanted to pass along the bug feedback in case it's not in the list of bugs.
Not a bug. Change your bond. PS4 exclusives dont work for cross save until this fall.