I could not place blocks repeatedly, especially when filling the last hole in a field after having to go to a chest to get more dirt.

After logging on and off several times I found out, that each time the place was occupied by a villager who did not show. Since he gave no sound and took no damage when I hit him, the space behaved like empty in any sense (you could walk through, ...) only you could not place a block. Even after digging out the dirt around, the villager would not leave the spot. So seemingly he got lost as an entity, but not as an occupied space.