I'd say the PRSOV is leaking.
If the isolation valve is closed (as proved by yourselves using APU air on the ground) and you're getting air on the right system , the only place it can be coming from is the No 2 engine bleed air.
With Bleed off the PRSOV should be closed but it's possible it's not sealing properly.
Doesn't really help with the original snag as you'll still be getting hot bleed air in the ducting.
Edit - CFM powered 737's normally have PRSOV's which are spring loaded closed. What did your MEL say about locking out the PRSOV?