I recently set up an iron farm, and used villagers from my breeder to populate it.
I went away to work on some other projects and came back to none of my 6 villagers and no idea where they could have gone.
This started about the time when 1.15.0 came out...
I have had to cheat them back in over 5 times and I don't like to cheat...
I have been going into creative to spawn them back in but I'd like to keep them spawned in the way they are...
I've traded with them, built a glass case around them, and watched them for a while to see if any mob could get to them. Nothing is working.
You can check the logs to see if they are dying, otherwise your information might be useful in MC-153904