Interesting data point on this. I booked an award ticket for myself from my son's account. Though I entered my MP number, it didn't stick, and the reservation showed my son's number (with my name). After ticketing, my number still couldn't be entered (could be entered but didn't stick).

I called MP, and the agent, a former UA agent, said right off the bat that she had been told that award tickets are permanently attached to the awarding account's number AND BENEFITS. While she later found out that she was incorrect and was able to change the MP number to mine, she reiterated that their training was that all award tickets would have the benefits of the originating account.