If a wither T has the head side towards a negative direction (north, west, or down), dispensers are unable to place a wither skeleton head where they are supposed to. However, they are able to place wither skeleton skulls far into positive directions that they should not be able to.

Steps to Reproduce:

1. Place T of soul sand with the flat part facing north, west, or down.
2. Place a dispenser with a wither skeleton head such that it would place a wither skeleton head on the top of the wither T.
3. Power the dispenser.

Observed Results:
The dispenser shoots out a wither skeleton skull.

Expected Results:
The dispenser places a wither skeleton skull.

In the second video I show an extreme case (I believe the most extreme) of dispensers being able to place skulls in places they should not be able to.