MH adopts "QF-style" award inventory. IME, there are usually 2 U seats on short-haul (<4 hours) and 1 U seat on long-haul. Once that U seat is gone, it's gone. But if someone cancels the U ticket, it goes back into the U inventory. They used to open up U before departure but they don't do that anymore.
Passengers using Enrich miles does not come from the P/U/X inventory. The miles required varies according to the booking class based on the
chart found here.