Agreed! It usually means you have a winning bid but they are trying their luck to get a sucker to pay a higher price at check-in. It happened a few times to me that I got a new boarding pass when I was about to get on the aircraft during boarding.

Funnily enough one could argue that it breaches the T&Cs of the upgrade since you're entitled to use the lounge but can't since you only get the privilege when you have to board your flight.