Skeletons that are riding a phantom move weirdly

Observed behavior: The skeleton riding a phantom circles in one place, mostly without changing its Y level, and in case it does, it usually doesn't move up

Expected behavior: The skeleton riding the phantom doesn't circle in place, and moves up and down

How to reproduce:

Step 1: Paste /summon phantom ~ ~ ~ {Passengers:[{id:skeleton}]} into the chat
Step 2: Observe