This last chest is also not dropping an orange item either.
Well, it does contain "orange" items (I've seen them) but I've also had runs where it only contains 2 "blue" items (probably what happened with you). In all cases, the items were level 33, where the rest of the items in the instance are level 37.
This must be a bug! I really cannot imagine why the last boss in an FP would *only* drop gear that is 4 levels below anything else dropped in the FP.