All villagers disappeared from iron farm.

Noticed iron farm suddenly stopped producing Iron Golems so went into the trading hall below and ALL villagers were gone. I had 20 villagers in secure stalls with all trades locked in and discounted to 1 emerald using the zombie villager curing technique. Beds are hidden behind the walls close to each villager. Trading hall is chunk aligned. The north row of the villagers was on a border but still in the same chunk as the others.