I was playing on my minecraft world with my friends when I noticed villagers dissapearing, I didnt think much of it because at that time I didnt build any protaction and maybe they wondered off, but then they started dissapearing very frequetly!!! We made sure to slip the night and we also plastered every square meter of the world with torches but they keept disappearing!!
That's until one of them disappeared in front of me, he was a snow villager that I brought trough the nether, at that point I gave up... Until my friend Who was coming home from a nether mining session found him in the nether and when he brought It in the overworld he teleported on the nether portal!!!

Hope this was of help🙏🏻