On my realm myself and my friends started upgrading a village. The village started with about 10 villagers. Over time the villagers started disappearing. 

At first we thought that mobs were killing them so we lit up all possible parts of the village, However, the villagers continued to disappear.

Consequently, the last two villagers we locked in their houses (no doors, well lit) and set an iron golem outside the house. These too disappeared (including the iron golem).

This also happened with a second smaller village.

This has been occurring since at least 1.17