Occasionally mobs don't render when in nearby chunks. This is most obvious in my villager trading hall where entire rows of villagers within a particular chunk won't render when viewed from a particular different chunk, either adjacent or adjacent +1. When you move out of the affected chunk into a different one all the villagers suddenly appear, they also make their honk noise as if they weren't just invisible but weren't loaded. I have found this also affects hostile mobs. The issue tends to resolve itself after a reboot but leaving the game and restarting Minecraft does not fix it. Another characteristic is that often the villagers who ARE visible appear with their backs to you no matter how close you are. I will attach a screenshot or two to illustrate.