That isn't the issue OP raised. It says first or business below both the outbound and inbound flights. The issue OP had involved a connecting flight, whereby the short hop was upgraded to FC but the connecting transconflight wasn't. The problem is Priceline isn't breaking down the flight by segments on the confirmation page after you choose the upgrade option, just by the overall leg so all you see is CHS-IAD-ORD with a single class of service listed. So according to the OP it is unclear that it could be a mixed cabin (I can't verify this, as I haven't been able to find an option that allows for an upgrade on only one segment of the outbound leg). In my testing, anytime a connection was involved I only had one option to choose for first class and couldn't get a mixed cabin. Also when I chose the FC upgrade it immediately put me to the seat map for all my flights, so I feel like something should raise a flag to the buyer if I'm selecting row 3 on one flight and only have options for 15 + on the T-con.

So seems like the real issue, is the checkout page is just looking at the class of the first flight of a leg, and not considering additonal segments when it decides what to display. Either way this is a priceline issue, not a UA issue. Expecting UA to honor the fare because priceline displayed it wrong is ridiculous, and the issue needs to be taken up with Priceline.