I used to have that issue back on older versions of the game (pre revamp/1.3) the loss of the save icon however makes it harder to tell when the game actually saves. Back then i would always open the menu multiple times, wait for the icon then go in and quit to menu.
I'd recommend playing around with how you treat closing the game, do some simple and cheap edits near your spawnpoint and experiment with actions before you close out, if you edit sticks you know it saved, if your edit disappears you know it didn't.
Do you experience a fair bit of lag/frame loss in game? (Like Freezing, or slow renders?)
If your phone is a bit overwhelmed it is possible that it can't process the game's request to write save data and therefore misses world based data.
If your issue is lag based than there are various 'out of game' causes that could be your issue, close as many active/background applications as you can. (Via Button left of home button)
Your internet browser could potentially have a lot of tabs open (similar to computer browser tabs) i know people that probably had more than a thousand tabs, mostly duplicates, it froze for a while when i closed all.
Also try looking in your settings for the built in device manager, using battery and memory cleanup prior to launching the game can free up memory for terraria to run alot smoother.
