HTC hasn't been doing the best in recent years, even squandering an attempt to revitalize itself with the U Ultra. Still, the former Android king is trying to keep its phones updated, even some of its older ones. Previously, several international versions of the One M9 started to receive Nougat. The U.S. is quite a bit behind, it seems, but it looks like the T-Mobile variant is the first to get the update.
Here are some screenshots of the v4.27.531.6 update prompt, with the file itself being 1.09GB in size.
Hopefully, more U.S. variants of the M9 will get the update soon. In the meantime, have fun, Tmo M9 owners.