Invisible mobs with no hitbox

I was playing on my private server, building a witch farm, and a skeleton came out of a cave, so I re-logged, and while the skeleton did visually de-spawn, it still existed, and continued to shoot me, even though I couldn't see it, and it had no hitbox. (tested with F3+B.) After it died to sunlight, I could pick up its items. This has also happened with endermen (unfair), zombies and creepers. Please fix this. The server is vannila 1.13.2.