I don't know if this is intentional, but Mob Spawners aren't full blocks. The autojump feature doesn't recognize this as being the case. This can be demonstrated through the following procedure:

1) Place a solid, full block on the ground.

2) Place a monster Spawner on top of that block.

3) walk up to the pillar you created and jump while walking forward 

You should end up standing on the edge of the full block below the spawner.

Again, I would like to reiterate that I do not know if this is intentional, so I apologize if I have wasted your time.