If you push solid block inside Wither Skeleton's head, visually Skeleton is usually moved, but starts taking suffocation damage as if he wasn't pushed by Piston.
And he's not pushable by non-solid block, though sometimes visually it seems like he is.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.