After the latest update (1.18.0 which released today), I am still having villagers and other entities disappear at random. None of the villagers were able to pass beyond a chunk border. They are housed inside a square, roofed off building they are unable to leave that sits entirely within a single chunk. I also had 4 minecarts used for a couple of super smelters nearby. Those also disappeared.

What happened leading up to this was the following:

I had played a relatively long, 3 hour session. During this time I had crafted 4 minecarts (as my previous ones had disappeared a few days before the update), and cured two zombie villagers. I housed the cured villagers within a building that is completely closed off from the outside, and sits entirely within a single chunk. The villagers are unable to cross a chunk border.

I bred the villagers twice before logging off, leaving two assigned villagers, and two child villagers. All had beds and work blocks. Immediately after logging off, I exported the world in order to back it up. The world is moderately large, approaching 600MB. I came back a few hours later, logged onto the world, and immediately noticed the villagers and minecarts are missing.