A villager with the exact same name being spawned in place of another villager with the exact same profession but a different skin and genetics, as well as their own family tree, and replacing the original villager while making the game think that original villager is still alive.
Finding original villager becomes impossible as since its entity stops existing entirely Yet family tree still keeps it considered alive, Villager Tracker freaks out, and Whistle simply not having said villager as an option.
It gets even weirder as since if said "doppelganger" dies, it gets no gravestone.
The original villager is still considered alive by the game, btw.
Upon searching with the villager finder, you can find the original villager in search; hower; it will either lead to the location where villager were before disappearing or the pointer of the item will freak out. (Last behavior happened after restart of server.)
More to add:
Said villager had 2 children, and all of its children as well as villager before disappearing were told to be staying while its "doppelganger" as far as I remember, were walking freely.

To Reproduce
Steps to reproduce the behavior:

1. Be in multiplayer
2. Get out of chunk where villager is
3. Somehow get this rare rng

Similar or exact same issue were already accuringwith other people https://www.reddit.com/r/feedthebeast/comments/1bykpox/mca_reborn_villager_disappearing/

Version
MCA Version: 7.6.1, Minecraft version: 1.20.1, Modloader: Forge

Logs:
Were unsaved becouse of server restart
The only closest thing i have is:

My fix for it were putting grave somewhere in my village and using command /mca-admin assumeNameDead (village name)
Just for people curious and then using staff of of life on a grave

Side effect of this "fix"
It were already broken so at least something to prevent softlocking