Was booking a TPE-SFO flight for my son, Expert mode shows PN1. After I bought the ticket and apply my PP, it shows waitlist only. Called GS desk, asking why and if he can confirm. Below are the conversation exchange:

Me: why I can't get a confirmed upgrade when united.com shows PN1.
GS agent: Because you used your miles to upgrade his return trip
Me: ??
GS agent: Was the miles for upgrading return trip his miles or your miles?
Me: Are you a GS agent?
GS agent: Yes I am a GS agent.
Me: Thank you, good bye.