So this is an issue that I noticed happening recently in all the 1.15.2 servers I play in. The villagers will have all the requirements needed to keep them happy; Food, beds, space to roam, and jobs. No mobs spawn in the area as I have everything in a 10 chunk radius lit up. I removed any blocks they could climb up on and fall off of, but yet they disappear if I don't trade with them for awhile.    I thought this was a one time bug, but maybe it's more common with other players? It affects servers on paper, vanilla and minor modded. (I also don't have any evidence as I can never catch it in the act, Very sorry)