mmWave is the cellular worlds equivalent of a halo car, it's great to show off but the practicality of it is on the lower end of the scale. Don't get me wrong , I think it's really cool to get 2 Gbps on mobile, but mmWave is a frail technology. They must have put a 5G node inside of the airport, given your speeds, it was very close by and it should have been easy to spot.
Not sure if it's still a problem, initially phones were thermal throttling when maxing out the 5G connection for longer than a few minutes and would drop down to LTE.
I prefer T-Mobile's layer cake approach with n71 and n41 for coverage and n260 and n261 for speed. Having a solid 200 Mbps whether it be on band 66 LTE or n71 NR is more meaningful to me than having 2 Gbps but having to be in a dense urban environment within eyesight of a node or macrocell.
Here's 3.5 Gbps in Vegas