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It's not just in villages.
It can happen to everything from livestock on farms to pet animals in houses to installed armor stands.

In the case of self-made bases, it often happens that a player goes off on an adventure far away and when he returns, some of the mobs have disappeared.
In the meantime, the spawn point is not fixed, as the beds are carried, set up, retrieved, and spent the night over and over again.

I don't know about the woodland mansions, but at one pillager outposts that was put off due to lack of preparation, the allay that was supposed to be in the jail was gone when I revisited it.

My own sense is that mobs and entities far from the player character or spawn point disappear at a higher rate and on a larger scale the longer the interval between visits.

This phenomenon happens routinely when playing Minecraft on the "Switch", regardless of play style or world.

I exaggerated the distance and time, but this phenomenon also occurs when the distance is much closer and the elapsed time is a few tens of minutes.
(In such cases, the number of mobs that disappear is also small.)
No game crashes or network disconnections occurred during this time.

Platform is Switch.
Utilizes realms, single and often multiplayer.

Each play session lasts 1-2 hours.

There are occasional forced logouts due to network disconnections, and sometimes you have to log out to run errands during multiplayer games.

Neighboring villages
It is about 300 blocks away from my house.
When I go there occasionally to run errands, several villagers who should have a transaction history are gone.
A few cats that were always with me are also gone.
Fences and lights are already in place to protect the village.

Distant villages
A village 1,000 to 4,000 blocks away from your home, a village you saw or stopped by while mapping.
A few days later, when I stopped in the same direction for work, it was uninhabited.
Not only were the villagers gone, but so were the iron golem, the cat, and the livestock inside the fence.
On my first visit, I went to bed at night and left there during the day when the weather was clear and there was no hostile mob.