I was playing and walking around the village to see if there were any zombies alive during the day (so they wouldn't attack villagers) and I came across a villager inside the leaf.

Points to note:

First point: I had been in the village for a long time and there were no trees to grow in that region.

Second point: As I said above, I was in the village for a long time and there was no villager trapped at the beginning, I can be sure of that because previously I had put more beds in the village and I had seen each villager sleeping in their proper beds (to have sure they wouldn't be attacked by zombies).

Obs: I had already noticed that some mobs are generated inside blocks of leaves naturally (like zombies, skeletons, vines, among others) and also if you plant a tree near a creature (like endman, Iron Golem, Wander, zombies, among others) but it was the first time that this happened spontaneously (or without me being sure how it happened). I put some pictures of mobs as examples of some situations (besides the villager that I consider a case apart).