Villagers start disappearing over time, I had a bunch of villagers once but are gone (in peaceful). 

I had a mending villager in a very safe place. I have an old and the one now version of my world. Clearly, most of the villagers are gone.

How to Recreate: Just have a nice big village, play near it, unload and reload. Just do things around the area.

It started in 1.13.0