Spider Jockey: Spider moves, but Skeleton not

What I expected to happen was...:
The skeleton should be joined with the spider.

What actually happened was...:
Instead, only the spider moved, while the skeleton seemed to stay on his position. (After killing the spider, the skeleton stood there normally.)

Steps to Reproduce:
Try to find a spider jockey. (Or add some method to spawn spider jockeys and then spawn them till the bug appears.)