Why are my Minecraft Villagers disappearing?

Me and my friends have fetched several villagers on a Nitrado Minecraft server in 1.16.5 and locked them in a fenced area, where they can't get out under any circumstances because of the fence gates. We gave them jobs, zombified them and healed them so that they could offer cheaper trades. However, villagers just disappear again and again, so that we had to re-zombify and heal them several times. Why is this happening?

As far as I know you have to give them a name with the name tag like animals. Because then they'll probably just disappear.

Nope. Name tag is only necessary for monsters and fish.

Maybe a zombie or something else spawned and killed her. Villagers can't simply despawn like that

Maybe that could be the solution. But we're actually pretty close to the villagers, and as often as some have disappeared, I'd be amazed that we never saw them get killed.