No, your connection status has nothing to do with filling in the flightplan form. I absolutely cannot reproduce the issue you are describing.

Here in my office, on Wifi, it works as advertised. At the airport Sunday, I was on mobile data and was unable to change POB or other information. I can only think this was the cause, a bad mobile connection maybe?
I still think it would be good to have an 'enter' or 'confirm' button on the sub-forms popping up when working on the flight plan.

I'm sorry, I still have no idea what you are talking about. On Android if you go into the Emergency Radio screen and change any of the options, a simple press of Android's back button is all that's required to get you back to the flightplan form so you can change other options.

There's no such thing as a long press on the Android back button, unless your device has some sort of bespoke "feature" which closes all windows when you do it. That wouldn't be SkyDemon's fault though.

What am I missing?

Yes, it is the Android back button on the lower left of the screen. I was on mobile data and it seemed impossible to enter data that would be retained. I think a dedicated back button on the flight plan sub form would be better. It should be insensitive to long/short press.
I had to spend half an hour to file a simple flightplan (number of POB had changed), in the end we called RR to get it out. Arrived at the home airfield with seconds to spare before sunset. Hefty surcharge after sunset on a weekend.

When you say the left arrow, are you referring to Android's back button? It isn't clear exactly what you're doing.

When updating the flightplan form, for example Emergency Radio, after entering ELT, I pressed the arrow left on Android and was thrown out of the flightplan form. When I do it at my desk, it seems to work better, at the airport, with mobile data, it was impossible to enter data.
I would prefer the sub-forms of the flightplan form to have their own 'back' button instead of relying on the left arrow of Android. If you press the left arrow too long you loose the information you just entered.