This happened to me last month with a ULN-PEK-SIN booking. PEK-SIN was showing HL (waitlisted) status in CA's system despite the fact that my ticket was correctly issued.

Luckily, there were still seats available on day of departure, so they just cleared the waitlist and let me board. If there were no seats, I would've been denied boarding without compensation since my reservation was incorrect.

However, the agent told me that if such a thing ever happened again, I should contact the Air China ticketing office and tell them that UA has already issued the ticket but there is a glitch so CA needs to clear you off the waitlist. Apparently, they can escalate it to Inventory Management to solve the problem.

UA will not be able to help you, apart from cancelling and rebooking based on current award inventory.