Villagers disappear/despawn?

I played minecraft for a couple of hours trying to find a village. The next day I brought 2 villagers to my base and started breeding them. From this, I got a Farmer, Librarians, and many more. In total, I had 12 villagers. I then encased them in a large room, with no outside access while being at a high light level. I then unloaded the chunks by going through the Nether to my Farms, and went AFK for 2 hours. After a successful AFK session, I went back to my villagers, hoping to do some trading. When I opened up the room, all I found was one of my villagers, the first one I got at the village. All the others vanished, leaving no trace behind (The Iron Golem that naturally spawned inside also disappeared).  I spent a couple of good hours trying to breed and filter through the trades to keep only the best ones (I found a mending villager, Luck of the Sea for a good price, and a great Thorns 2 price, and many more). Now all that hard work has gone to waste. What should I do?