Similar scenario. I have a reservation for two rooms (a colleague and me). I’m Titanium, he’s Titanium. I call to add his name to the reservation and his Bonvoy number. Everything is going along ok until they ask me for the last four digits of HIS credit card number. I’ve already provided his name, address, phone and Bonvoy number. I kinda chuckle to the agent - “Nope! Don’t have that one!” Well, hence commences my ridiculous spiral. If I can’t provide the last four, can I tell them the last Bonvoy hotel he stayed in? (No). Ok, the phone number on the account prior to the currently listed phone number? (Hard no). Ok, the address on the account prior to the current address? (Told her he’s been at his home for 30 years, long before “Bonvoy” was a twinkle in anyone’s eye.).

And that was game over. They wouldn’t add his Bonvoy number. What nefarious thing were they attempting to prevent? I wasn’t trying to claim his points - I was trying to be fair!