Wither skeletons don't drop skulls when killed by tamed wolfs.

Making use of the mechanic at a large scale in our sever's nether farm, tamed wolfs do indeed kill wither skeletons. This results in drops such as bones, swords and coal, but no wither skeleton skulls. XP orbs also appear, confirming the kills do count as player kills.

While the drop percentage is low, the issue has been tested over several hours, with no drops appearing after hundreds of killed wither skeletons.

A quick pass over spawning wither skeletons with a sword does provide us with drops, showing the overall mechanic does seem to work.