The story 

After playing for a while my village is population increased to it is maximum value (around 15-16 villagers) and when i traded with my lvl 4 farmer villager the game crashed and when i reloaded it and the map that villager disappeared. I thought he is just stuck in a wall or something like that but when i traded with an other farmer the game crashed again than i realized half of the villagers in my town disappeared too. The farmer himself and other 3 of my farmers, 2 masons, a weaponsmith, 2 toolsmiths and a librarian disappeared. I started to search for a sinkhole or a cave maybe they fell in there but on that day (in minecraft) 3 baby villagers were born so it means the beds which were occupied by the villagers who disappeared became free. This is why new villagers were born so it proofs that the old villagers disappeared completely and i not just can not find them. They can not be killed by pillagers or zombies because i live in the town so i should know about it and there are 2 iron golems in the village, which is not big, and anyway this mistery happened under 2 minutes.(reloading the game twice)

Reproduce

After i tried almost 20 times i found a way to reproduce the crash. 

1. Trade at least two times with a farmer villager who at least level 3
2. Go to an other villager who again at least level 3 and trade with him 
3. If it did not work repeat the whole process...

Notice 

**The villagers disappear full randomly after crash.