yes if you have a good clear wifi signal then it should stay on UMA until you move out of range of the access point.
Do you have more than one wifi network configured on your bb from signals near your apartment (like any neighboring networks)? If so, you might want to make sure you move your primary network up to the top of the priority list. Not sure if that has anything to do with it but it seems like when you turn your mobile network back on, it is for some reason thinking the wifi signal is not an appropriate option.
Tmobile | 7100t, 7105t, 8700g, 8320, 8900