Right, with PMUA that was my experience. If I bought the upgrade before check-in it booked into Z and was only offered if NC>0.

But this time I bought an upgrade for mid-April and it booked into PN. I did not buy the upgrade at booking but immediately after the reservation was ticketed. Is the fact that R/RN=0 the reason? Or would it have still been PN if R/RN>0?

Looking at another post I see that code PN translates to a B/M-Up for Elites and that it tracks to the P bucket. The P fare is a discount business fare that's supposed to give 150% PQM.