My understanding is that once you are on a call, whether you made the call or you received the call, and whether you are on WiFi Calling or your carriers network, a call will not switch from one to the other. Your call will be dropped when you lose the connection you started with. I understand that this is true for all carriers and all phones. Also, less well known, if you start a call on LTE and you travel into a 3G only area your call will be dropped.
I have the same problem on my iPhone 8. I’ve been on hold for hours without a solution, other then this. Switch on Airplane mode when you are home and that forces your phone to stay in WiFi calling. It is a poor solution requiring you to remember to do this manually when leaving or coming home, and no substitute for good software, but since Xfinity Mobile won’t staff up their tech support to actually handle their inbound calls, it’s all that I know to do.