Have searched, but can't seem to find this same bug, but please tie up to another if there is already one the same listed!
When exiting game in usual steps (pause button > Quit to Title), and returning to game later in day, all progress (a LOT of placed blocks) had been lost, but usage of tools was the same i.e. it showed all the use the diamond pickaxe from mining, but none of the new structures built were there.
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Duplicate of MCPE-12347
Thank you for your report. This issue has already been reported at MCPE-12347.
You will see from the comments there that this issue is under the attention of the developers.

Thanks, but this isn't a crash causing the rollback. This is happening upon exiting the game in the normal fashion, without any crash occurring.

Thanks, but this isn't a crash causing the rollback. This is happening upon exiting the game in the normal fashion, without any crash occurring.
Please read the comments of MCPE-12347 as well as the other duplicate tickets - You will see that that bug report encompasses several scenarios where the game progress is lost, including exiting the game in the way you described.
Please read the comments of MCPE-12347 as well as the other duplicate tickets - You will see that that bug report encompasses several scenarios where the game progress is lost, including exiting the game in the way you described.

Ok, thank you!

Ok, thank you!
Duplicate of MCPE-12347
Thank you for your report. This issue has already been reported at MCPE-12347.
You will see from the comments there that this issue is under the attention of the developers.