https://wccftech.com/annoying-ios-13-bug-disconnects-iphone-11-from-wi-fi-switches-to-lte-while-on-facetime/

Is your iPhone 11, iPhone 11 Pro disconnecting from Wi-Fi and switching over to LTE during a FaceTime call? You're not alone, at all.

Ever since I upgraded to the iPhone 11 Pro Max, I noticed an interesting little bug whenever a FaceTime call came in or if I was making one. Whenever the call connected, I noticed that my iPhone disconnected itself from Wi-Fi and switched over to LTE. When I ended the call, Wi-Fi would just connect again on its own. I tried restarting my iPhone, installing iOS 13 from scratch and made sure I didn't restore from a backup after installing iOS 13, but nope, nothing helps.