I think those are "official platinum" tickets, which tend to be the seats that are dynamically priced through Ticketmaster for concerts. For Ben Platt's recent concert in NYC, some of the best seats that weren't VIP were tagged as "official platinum" tickets and they, too, were pricey a month or two before the concert (some evern more expensive than a VIP ticket.) As it got closer, though, prices decreased but I don't think it was at the same rate as you experienced.