Villagers cured from zombie villagers disappear when others are cured

I had beds and work stations set up inside a floating, excessively well-lit, and sealed concrete building above an ocean where I was luring zombie villagers from the ocean floor up to cure one-by-one and waiting for them to be cured before naming them and sealing them inside the perfectly safe building. I had cured and named one so far, but when I returned with the second zombie villager, the first that was cured was missing. (It couldn't have been zombified by a drowned, because the building was perfectly sealed as well as being raised above the ocean, plus I would have found it when I returned if it was zombified by a drowned because it was named and therefore would not despawn. So I decided to get another zombie villager which I waa able to cure without issue and begin populating the village building. I decided to make an addition to the building that would allow me to zombify villagers safely with a trapped drowned in a minecart, and then cure them for cheaper trades, but whenever I converted and cured a villager, villagers that I had specifically cured, named, and leveled up through trading went missing whenever I left the area and were replaced by level 1 unnamed villagers of the same profession, as if they had died or despawned and were replaced by new villagers. I don't think screenshots would be helpful, seeing a picture of a named villager and then one of a new villager would be very useful.