Same story here from my sailings on Liberty OTS in December. They had someone at the entrance to the Diamond evening cocktail reception checking seapass cards. If they didn't have Diamond or higher level printed on them the person was not allowed. As in Cruise Fantatic's case we had several hundred Diamonds aboard and crowding was a problem. Like me, many booked to get double points in December.
Crowding is becoming a problem on many cruises as more and more people obtain Diamond and Diamond Plus status. I predict that once they have installed dedicated Diamond lounges on all ships they will restrict the Concierge Lounge to only those in eligible suites and Pinnacle members, and Diamond Plus will have to use the Diamond Lounge. The whole thing is a jumbled mess. On Liberty Of The Seas you can access the Diamond Lounge from the back by just walking up a set of steps and walking right on to the little patio, and then through the sliding door. No key card reader involved. There is a swing gate saying Diamond only, but it isn't locked down.
When the concierge or diamond host is in the lounge, as soon as someone key cards in at the door their cabin info and security photo pop up on the host's computer screen. They know who is supposed to be allowed entry and I think this feature is to keep people from using someone else's card. On Enchantment last summer the concierge greeted me by name the first time I entered the lounge. I wondered how he knew me.