There are currently some right-click actions with shears on mobs that dispensers containing them cannot do, namely the following:
Removing pumpkins from snow golems
Shearing red and brown mooshrooms
Dispensers also cannot shear placed pumpkins, however this is covered in a separate issue which has been marked as WAI (although not by Mojang, so it may be worth looking over); see MC-148448.
Since the reporter is inactive, can I request ownership of this ticket?