I have only tried to use the Zwift companion app twice, but both times I have lost Bluetooth connection from my trainer mid ride. This also has never happened when I didn’t have the companion app open and I have tried rides in between the two uses of the companion app and after and they have been fine.
What device are you using to run the Zwift game itself on?
I’m using an iPad
Have you tried the Companion app with Bluetooth disabled on the phone?
using iphone se found this happened when battery fell below 60%, try plugging into charger .