This may happen due to the exotic lifecycle of the app on your OS. Normally the game should remember there are unsaved changes (new research completed / some items gained) and save them after some time or if the game is properly closed.
Your items (both resources and tiles) should save in snapshots, there should not be a situation when one thing is saved while other's not.

I had no luck reproducing this issue, and see no easy workaround for it - just try not to quit the game immediately and do not kill it through the Android's recent apps menu (basically you should never do that with any app unless it does not respond to your actions)