All villagers and animals suddenly vanished upon loading saved game

I've been playing the same saved game for months.

Since 2 days ago, every time I load my survival game all of my many villagers have disappeared or failed to load - permanently. The village was dead.

I first assumed they were all killed by zombies but after repopulating the village by borrowing villagers from another village they disappeared again the next time I loaded the game. The same applies to animals.