Had this issue yesterday and had not read this thread before it happened. Came up in red "Aircraft Disconnected". This is where I was glad that I wasn't flying with my old iPad obscuring the transmitter because the initial panic of me thinking I had lost TRANSMITTER connection was reduced when I saw on the transmitter the rotor RPM and other data was still functioning. I tried closing and re-opening the app etc with no joy, so I hit RTH on the transmitter and all was well. However, after landing as I disconnected my Nokia 8 and returned to the home screen I saw that an Android update had been occurring in the background, which I think may have been the culprit (although I had deselected WiFi I hadn't put the phone to airplane mode). Lots of lessons here I reckon:
1. Fly with phone solely focussed on the drone. Close all other Apps, use offline maps and turn off both WiFi and cellular.
2. For those who say that "all the data is on the connected device so you don't need to see the transmitter" it's worth remembering that it can certainly help to see it in situations like this (and you'll need it to RTH).
3. It was a good reminder for me to check that the drone would RTH if the transmitter signal had indeed been lost.
4. The "Aircraft Disconnected" message is a tad worrying and could be changed by DJI to say something like "Device Disconnected use transmitter".
This may not help those who suffer this repeatedly regardless of what settings they have on their conbected phone/device, but my hunch is that in my case the Android update spooked the Go app, which disconnected itself.