A number of villagers seem to disappear.

I have a village in a superflat world, with villagers placed in a line, each with a bed and workstation, the villagers also being separated by a solid block. One line of villagers is standing on the bedrock floor, another line is placed above them, with one solid block as floor separating the two lines.

I discover that a certain number of the villagers have disappeared. The remaining ones are allowed to breed, and soon baby villagers refill the places of the disappeared villagers. Once grown, they take up work. After a time, the new villagers in exactly the same spots vanish as well. No other villagers disappear than those standing in those spots.

The second line of villagers, standing on the floor just above the disappearing villagers, do not vanish either. This happened twice or thrice in a row, with only those standing in specific places vanishing.

There is no way monsters could have gotten to them.

I did not see when they disappeared, it could have been while sleeping. It is odd that  not only did a number of villagers vanish, but it  happened twice, to villagers standing in the same spots, with no others than them disappearing.

I cannot figure out how to reproduce this.