Looks like CellMapper is still either extremely backlogged or having issues mapping NR coverage. I went through Nashville twice over a week ago and none of my AT&T NR mapping is showing up from that entire trip. It was from two different sessions, I went down to Linden TN on Friday afternoon and came back on Monday. Uploaded all points when I got there Friday. Looks like my T-Mobile mapping is showing, I don't think there was any NR showing in Nashville previously. I do have the paid subscription for Cellmapper, which is supposed to prioritize uploads over non-subscribers.
Your area must be unique. I NEVER see dense T-Mobile coverage. T-Mobile is actually the worst carrier by far in my eyes. They normally do not know what dense means. To make a good network, it is going to be very very expensive because they are starting with a very poor network. T-Mobile was a great marketing device, but not a great cell phone carrier.
Here's another good source I've seen from the user lart2150 on reddit posted on the T-Mo subreddit: https://coverage.lart2150.com/# They use the T-Mo map and scan it to take all the info you can see in specific spots and show it nationwide. It shows pretty clear holes in the Madison market, it's definitely not great. Meanwhile back up here towards the cities I think it's probably one of their better markets.