In a rather creepy happenstance, my colony found itself missing two colonists when they didn't show in the mess hall (found tables and beds unassigned). Later, I saw that the number of colonists had decreased from 8 to 6. I've tried without avail to find the corpses, thinking their friends might need some closure. Turns out that the others didn't seem to remember them being there in the first place (no stress due to "death in colony," supposing that's a thing). In fact, the increased elbow room seemed to have lightened the overall mood, eerily.
Deep fried mush bars at the tail end of preparation. Lice loaves on the floor where they had left them (so unhygienic)!
After noticing the problem and saving/reloading the game, the whole thing crashed after about a minute. I submitted a crash report. I was going on my 23rd cycle. There was a 22-cycle save where I still had 8 colonists. It seems likely that the evaporating colonists had something to do with the game crash. I'm pretty certain they didn't disappear simultaneously, but the second followed the first in short order.
This happened shortly after I refused a duplicant print for the first time, but really, there's no directly visible correlation, there, and I have no idea what caused this bug. I HAD been sending them into my CO2 basement to dig and lay tile, as well. You might find more helpful information if you can connect this bug report to my game crash report, which I submitted from within the game client around 11:30 EST 2/19/17.