Bug seems to occur randomly, blocks certain areas from getting into them. They vary in size from 1 chunk and can spread (I think) throughout the entire world. They are impossible to cross via underground. Mobs which appeared to spawn inside have locked animations. Barriers tend to dissappear upon world reload, although after reload game once spawned me high above the ground, resulting in death from falling. 

Below you can see blocked areas marked with holes on the areas' borders.

It's not the first time the bug occurred.