Villagers disappeared after reopened the game

So I had a building with five villagers in it. That time, I was afk to go for a reason and left my game on. When I went back, my phone off and the game was closed by itself. After I reopened the game and went to see my villagers, there was only one nitwit there and all of my worker villagers gone.

I've searched through people's reports, but none of it similar to my issue. I think my game was crashed and closed without I hit the save & quit button, but it never had been like that before, and I still can't figure out why my villagers disapeared.

Here I have attachment.

The villagers pic I took from my copied survival world and changed it to creative for an experiment few days ago. Then the issue occurred and left only the nitwit. 

I don't know why would this happen to me, therefore please solve this issue. I would really thankful.