I’m glad this has been brought up, my dad encountered a similar situation in late-April in BNE. He’d just got his new PS-chipped-card so I decided I’d show him the works by getting him to make that pole green in the airport, but it didn’t work as I’d given him an exit row at T-80, so I asked a handler nearby and they said they’d check him at the gate, no worries.
Went to the J lounge for breakfast (he’s CX Diamond) and didn’t think to get a boarding pass, as I wanted to show him he could use his card to board. Eventually got to his turn in boarding and to use a phrase from this thread, created a logjam as the agent couldn’t verify he could walk

and had to call the desk in the centre of the airport to take him off the aforementioned list.
Overall, it just seems silly that it can’t be done at the gate, and annoying that we were told it would be when it clearly wasn’t, however I do now know to see someone in the lounge about getting me off the list.