Default APNs switching?

Starting about .. oh, 3 or 4 months ago, I've noticed my phone dropping data, OR, getting stuck on 'E' or 'G' symbols, even in areas that have LTE. Every time I notice this, I go into APN Settings, and I find that the selected APN has changed. (Usually from 'T-Mobile US LTE' to just 'T-Mobile US'..)

I don't have any APN apps or anything, but have been running AOSP v4.3 (then v4.4) since about the same time this started.

Any ideas on what to check? Am I going crazy? (Is my phone going crazy?)