Steps to reproduce:
1. Connect to a multiplayer server with another player.
2. Make up a free slot in the hotbar and select it.
3. Get in a minecart or on a horse.
4. Dismount, then get your friend in a minecart or on a horse.

What is expected to happen: Your hand's placement changes when you ride a transportation device, and doesn't when another player does it.
What is happens: The opposite: hand position changes when another player rides something, yet doesn't when you do it yourself.

The 4th point is not reproducible in singleplayer but 3rd is: hand placement doesn't change when you ride a minecart or horse.
The issue might be related to MC-1349.