Do you mean that when you loaded you only have 9 left, or did they all leave after you loaded?
We're looking into it, but this is the first we've seen of this bug. They cant leave unless you are active on your farm, the only thing we can currently think has happened is some sort of corruption or editing of the main save file.
Again, massive apologies you've had this experience, and Im happy to sort you out with a boosted save if you contact me directly via contact@trilumstudios or join our disc and hit us up there.