To reproduce:

• Stand on a block with regular blocks around it (so normal ground)
• /summon zombie ~ ~ ~ {Passengers:[{id:"skeleton"}]}
• Kill the zombie.

Result: The skeleton moves to the a seemingly random side before landing.

This is not only a pretty random thing for it to do, it also often breaks the workaround for MC-145113, making it even more difficult to use one-command contraptions.