AT&T here with 7Plus. It happens to me to- but I notice it gets stuck more if I am using it while it loses LTE connectivity rather than losing it while its sleeping. Let's say I am in a building with poor connection that is like 1 bar 4G. If I step outside (where I know there is 5 bars LTE), it stays stuck in 4G. But if I am on the train in between stations and don't use it, it connects to LTE once I get to the next station.
AT&T here - Black phone. No issues at all with LTE under any circumstance.