villager behaviour canceled becaues of wall/fence and slab

In my singleplayer world I am trying to build my own village with villagers and a lot of details.

That is why I place fences and carpets to imitate a table. The room is 3,5 blocks high.

After I got my first villager he acted pretty normal (blastfurnance villager).

If this villager gets on the "table" he stop doing anything (still tradable).

In a testworld I did some testing and the results are pretty interesting.

If a 2 block high mob is on a fence or on a wall and if four slabs are placed above(like in the picture) it will stay there. 

villager: wont move, will be targeted by hostile mobs but melee wont be able to hurt him

zombie: wont move, wont hurt player or mobs

creeper: wont move, will explode if player is close enough

range mobs(e.g. skeleton): wont move but attack player...