Was this a full length or partial uq? If partial, you may have needed a pad under your feet.
There are a myriad of other things that can impact the lowest temperature an uq can go. Bomber mentioned several. I would add wind, clothing, additional layers, cold/hot sleeper, tarp pitch, hot water bottle… The list goes on, but I don’t think a 20* uq should fail at 50* even without any of these additional factors as long as the fit is tight. This seems to indicate an air gap to me.
There's a reason we call this CBS, because the cold is coming from underneath. It’s almost always due to either insufficient loft for the ambient temperatures (unlikely in a 20* uq with 50* ambient and little wind) or an air gap/bubble. I would make absolutely certain that the uq is snug up against the hammock, not just at the ends but also along the entire length. This cannot be ascertained while the hammock is empty. You need someone, preferably yourself, in the hammock to see how snug the fit is when the hammock is under load. Also, make sure that the down isn't compressed. You may have to give it a good shake-out before hanging the uq.