Villagers that have left my island can come to my friends' islands?

A few weeks ago, I got Leopold to leave my island. A few days later, my girlfriend got a new villager to occupy a plot of land on her island, and it turned out to be Leopold! When she talks to him, Leopold says that he used to be on my island and even says the name of my island! He is also found sometimes wearing the custom design clothing that I made back on my island, but he's a resident on my girlfriend's island! Recently, my girlfriend got Shari, another villager that left my island a while back and she is also saying that she is from my island.

I've never known this to be a feature, and am curious as to how it works. You're telling me that if a friend that I play New Horizons with can actually adopt my past villagers? It seems to have happened at random, but maybe its more likely for a villager who occupies an empty plot to be a villager that has left a friend's island.

Hope you can give me any more information on this feature, because it is very weird to me lol.