OK, here's an update since the month is over.
Phone measured data usage, 30 days: 3.44 GB. T-Mobile's measured data usage: 4.7 GB. (That's 37% more than the phone measured.) The numbers would have been much higher, except I had data turned off most of the last 2 weeks, and listened to downloaded media on my commutes.
I had contacted T-Mobile 3 times during/after the month, they told me to check allow data roaming in settings, and suggested watching what happens next month. I had also contacted a few reps at LG, but each deferred me to T-Mo, saying it's T-Mobile's software(?). I assume it's GMS Android or LG software on a Nexus 5.
At this point I'll wait to see what happens this month, or do a suggested hard reset. Thanks for reading and your suggestions.