So my friend (gamertag: AghastDig176518) and I started a survival world about a year and a half ago. We started making a village full of villagers for trading and just overall a cool area for our world. And ever since then our villagers keep de-spawning. We always name them with a name tag and give them a job block and a bed. They also have a huge house to live and roam in. We also always go on our world quite frequently. So for about as long as we've had this world we've been dealing with this problem. I decided to finally report this because I went on today and at least more then half of our villagers are gone again. It finally hit me that I need help with this issue because I'm tired of this happening and it's making me very frustrated every time it happens. 
More information: We rescued all of them via curing a zombie villager. We have a house for each designated job. Each are very well lit and closed off to protect them. There are also a couple berry plants near the back of the house to kill mobs from glitching in or something. As of right now we have 14 librarians and there are only 8 right now in the librarian house that I made for them. We also lost 2 farmers out of four, 2 butcher's out of 6, 1 cleric out of 3, 1 fisherman out of 2, 3 armorer's out of 4, and a Fletcher. They are clearly gone and like I said, this isn't the first time this has happened.