Just an update.... I was in full USCC territory for 4 days with 2 of these phones. Have been to the same area many times in the past without issue. This time, however, neither phone connected to any LTE roaming, none zip zero. Prior to this trip I would connect primarily on USCC with some AT&T here and there the entire time.
I tried rebooting, toggling airplane mode, changing the radio to "LTE Only", making sure Data roaming was enabled, power off/on, a zillion times.
Is this a 9.0 Pie issue? Sprint issue? How do I report this to the "correct" people?