Before leaving the casual game, did you click End Turn? And did you see something like, 'You will receive an emil notification when the next turn starts?' After that, did you leave the game by clicking on Exit and then click on Go To Lobby? If you hit any other buttons, like Abandon or Surrender, then you will leave the game (I'm sure you know this already).
Aside from these things, I can't think of another reason you would've lost your progress.
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The bitterest truth is sometimes better than the sweetest lie - Griffin, MIB III