I've also encountered problematic behavior using G2G on my Android phone (as I am doing right now).
My main challenge has been that the ability to edit or add to the end of a comment goes away as I get to the bottom of the edit box. If I plan in advance, I can defeat that by inserting blank lines after the comment, before I start typing. If I forget to do that, sometimes I just save the comment, then go back and edit it.
The answer box is longer, so I don't collide with the end of it nearly so often. I've never been completely unable to save an answer, though. I don't recall what I've done to evade the problem you describe; I think the save button might reappear if I move up to the middle of the box and start typing there.