Disapearing Villagers

I have some villagers roaming around my base(I'm in peaceful mode). Suppose to be nothing could happen to the villagers like being killed by mobs or something. When I leabe the game and came back with missing villagers. I checked the beds. I remove all and replaced ed them, I have 10 villagers before I left, I came back with 4. The 4 beds showed greens particles equal to the visible villagers left. I think its a saving error or something