Sorry if this is a duplicate. I searched 'Villager disappear' and found no results related to servers, only other issues where villagers disappear under different circumstances.

I restarted my snapshot server today to get the new one, and my villagers were gone. There's no way they could have died. Upon further inspection, it turns out that my villagers vanished because I installed the new snapshot!
However, it only seems to happen when updating, as villagers created in 20w07a stay in the world.