Part of me wants to say that this is the expected behavior as this is the same thing you would see with T-Mobile Wi-Fi calling when switching between the cell network and W-Fi calling. I have not seen or heard of this issue occurring while using DIGITS so I would like to get this up to the engineering team for further clarification. I am going to send you a private message so I can gather some necessary information from you.
My phone does the same thing. I turned off LTE and the phone no longer drops calls when switching from wifi to G4 and from G4 to wifi. How can I fix this so I can turn LTE for data only on again?