Note that the OP technically never received an upgrade. Most likely a day before check-in the system showed one as being available and the agent mentioned it. The OP interpreted that as guaranteed. And it is very possible the agent thought it was very likely to still be there when the OP checked in and led the OP to think that as well. Unfortunately unless they actually pre-upgrade you, it isn't guaranteed and in this case someone else got it first.

I did mention a one time situation of losing an upgrade after check-in, but that was a one-time in about 30 years of traveling. It was due to my checking-in very early, a very new and inexperienced front desk agent who didn't check me in correctly. But whatever he did caused my reservation to no longer show as still needing to check-in. So their system showed the room as available, but didn't show my reservation for a suite room. So when a Plat checked-in later in the day and asked about a suite upgrade, they gave it to him.

Also note that I did work out a resolution with them, though I didn't get a suite room that trip.