That''s really weird. It was my understanding that all Band 71 sites are n71 capable. Maybe T-Mobile feels that they don't have sufficient backhaul to turn on 5G on the site? Though I don't see why that would stop them since it would still improve network performance for everyone if it is live.
While I haven't seen a site like this in person I have seen on T-Mobile's "Personal Coverage Map" that there are some areas that have 600MHz LTE but don't have 5G service at all.