I also reported this ingame, but I wanted to share this here in case people wondered why that happens. We tried around a bit and it seems you can only see other players sit if your character is standing very close when the animation takes place. Otherwise he/she is just standing in front of the chair. We tested this with different characters and settings and with Addons on and off. So far it's a 100%. To clarify: it's about characters taking place on furniture like chairs and benches, not by emote.
Should you wonder why this is even important, well, it is important for RP and apart from it, it's not working as intended. A fix would be lovely. Thanks.
EDIT: I was mistaken! It's not the distance! It's the "line of sight". If there is any object in the way between one character and the other, the sitting-down animation doesn't play. The object doesn't have to block sight, but only has to be in a straight line between characters.
Edited by Valorin on February 15, 2017 8:30PM