If you open a 1.14, 1.14.1 or a 1.14.2 world with villagers in 1.16, the cease to exist.
Tests I did:
Created a 1.14 world and opened it in 1.16 caused non-traded villagers, including unemployed ones to vanish, the ones with trades are still in the game.
With a 1.14.1 world, caused unemployed villagers to vanish, the ones with professions are still in the game.
With a 1.14.2 world, caused non-traded villagers, not including unemployed ones to vanish, the ones with trades and the unemployed ones still in the game.
IWith a 1.14.3 world or a 1.14.4 world, the villagers were safe.
How it was different from what I saw earlier watching a streamer's live, I think the behaviour is not consistent.
How to reproduce the bug:
Create a world in 1.14 versions, spawn some villagers, trade with them and leave some without trading.
Save the world a and open it in 1.16.
Note that some villagers are not there.
How to avoid the bug:
Open the world in a unnafected version (1.15.2 is safe to do it) before upgrading to 1.16