This weekend I tested file sharing in flight using a Pi3 and Stratux 8.2 with a newer .sh update (not sure which one; it's still in the plane). I was able to share plans between my Android tablet and my Android phone, in both directions, without issue.
When I had the problem before, I could switch out my EdiMAX wifi dongle on the Stratux for a different brand dongle (TP-Link), and flight plan sharing would work without any other changes. No one else at that time reported having the same issue, so no one really looked into it. Switching dongles was my "fix". Looks like you have fallen into the same sinkhole I was in.