I'd like to refresh this very old thread. T-Mobile still refuses to fix this issue. I am now on a Galaxy 23. The same thing has been going on 4+ years.
They still "automagically" change this setting to Cellular Preferred. They have no business ever touching this setting. It is simply a customer preference. A preference should have nothing to do with what is actually happening while a phone moves.
What should happen (obviously) is that when a phone move to a location where there is no Wi-Fi they should (silently) change over to the cell network. IF the customer has Wi-Fi preferred and they go back to a location with Wi-Fi they should silently change over to Wi-Fi. There is absolutely no need for this user preference to ever change unless the user changes it manually.
I have tried to explain this simple concept to T-Mobile reps on at least 7 occasions (including over an hour today). For whatever reason they are completely unable to understand this simple concept. This is very, very frustrating!
ONCE I actually got a rep who understood the issue the first time I explained it. She finally said she'd refer to up the line. Obviously nothing happened (in 18+ months).
I don’t understand why T-Mobile is so resistant to fixing this issue.
BTW, I know that Verizon does not do this. I don't think AT&T does either, but I haven't tried them.
