The Shulker only has a radius on it's x and y axis and not the z axis when using something like @e[type=Shulker,r=5]. Well at least the top. I don't know if this was intentional but just throwing it out there. Maybe the Shulker can't see or attack you from the top?
MC-85189
Shulker radius doesn't apply when on top of them
Resolved
Fixed
0
Fixed somewhere between 15w32a and 15w44b, shulkers now even attack players on top of them.