The title explains it. If you sneak, open the player list menu by pressing tab, and try to walk left, it doesn't work. You can walk right while sneaking and having the player list open, but not walk left.

 Additionally, you cannot open the player list while sneaking and walking left.

Steps to reproduce: Sneak, press tab (or whatever key the player list is bound to) and try walking left. It doesn't let you walk left.

I don't know if this only effects environments where you're able to open the player list. I didn't test in Singleplayer.