Basically what I think is going on is that there's a certain distance where a villager can detect a bed, but the villager that's already assigned to that bed is not loaded in, so the villager that's several chunks away can steal that bed from them, and the original villager looks for another bed, which often ends up being another bed they don't have access to. I think there are several glitches that causes this to happen, the first being that villager shouldn't be able to assign themselves to beds from several chunks away. They also shouldn't be able to assign themselves to beds they can't path find to. Worst of all when they do assign themselves to a bed that's several chunks away that's not even accessible to them, they don't give up trying to get to it and will walk along walls and stand in corners, and just stay there. This just makes it really hard to work with villagers, I have at least 70 of them in my base, and dealing with them is such a pain.