As for the mythic multiplayer emote reward (chest):
When I use the MP Emote (The last in the mythic reward box (pvp)) on someone else, they get the prompt that they are invited. When they accept they move to the correct position to initiate the event. Once they are there, the emote fails to play.
So far, nothing special.
The additional bug - aside of the emote not playing - now remains in the fact, that the game thinks the other player is currently doing an emote. They can walk around etc. perfectly fine, but because the emote never played and resolved the game never got the info, that the emote finished.
That means that once this happens, the other player will NOT receive any invitation prompt for MP Emotes any more, and they can't initiate MP emote themselves as well.
I do not know if this is a universal bug or linked to other factors, but it essentially allowed me to disable anyone within the guild hall to perform any MP Emotes together and was reliably verifiable without failure.