Villagers Despawn After Server Restart

My friends and I share a survival world and use aternos to connect to it along with using a few plugins. I recently made a villager trading hall, complete with many kinds of villagers. I spent a very long time working these villagers up to their highest levels. I went exploring for cave systems and start loading chunks. After the lag wouldn't go away, I figured I should restart the server since that usually fixes things. I get back on the server and all of my villagers have despawned, except for the farmers for some reason. This is the second time it happened today, the first being when aternos force-restarted my server to fix things up I guess. I didn't think a regular restart would change anything, but apparently it did. Why are restarts causing my villagers to despawn and wasting an entire day's worth of work? No point in even trying to do it in survival if I will have to respawn them in creative mode everyday anyways.

Tried that after the first time they all were despawned. Same thing happened. Either way, villagers aren't meant to despawn naturally in Minecraft like other mobs.