Reproducible 100% of the time in Overworld and End (does not reproduce in the Nether).

Wither_skeleton can't move out from under end_stone, no matter how high above him the end_stone is. You can see in the screen shot, the end_stone above the pictured wither_skeleton is higher than the clouds, but still, this wither_skeleton is trapped under it. When trapped by end_stone in this manner, the wither_skeleton will rapidly chop his sword up and down in a "glitch" manner without being able to move beyond the boundary of the end_stone above it.

Breaking the end_stone block above the wither_skeleton will free him, and then he moves about according to his AI once again.

Wither_skeletons can freely move to place themselves under end_stone, but once under, they are forever bound under the end_stone so long as that end_stone exists.

Even more curious, once bound by end_stone in this fashion, placing any other blocks (tested with dirt) above the wither_skeleton between him and the end_stone above him won't free him, but, if you extend the intervening blocks (in my tested example, dirt), it will allow the skeleton to move out beyond the boundaries of the end_stone to the extent of the new boundaries created by the new intervening blocks. Yet, to actually free him still requires breaking the end_stone blocks that were above him and which bound him in the first place.

This effect is seen both with name tags on the wither_skeleton and not.

This effect is seen both with wither_skeletons naturally generated in Fortresses by the game, and those generated using an egg.

This effect is reproducible 100% of the time on Minecraft Realms and in Singleplayer.

This effect was originally observed in the End on my Minecraft Realm, trying to bring a wither_skeleton into an End City, and the wither_skeleton became trapped every time he moved under any end_stone above him.

This effect was then reproduced 100% of the time in Singleplayer both in the Overworld, and in the End. Curious, it does not work in the Nether. Wither_skeletons will move freely out from under end_stone in the Nether.