Bug seems to have been introduced since the Assassin's Creed crossover event and as of typing this message, still not fixed. As title says, any and all facial expressions/ animations that characters do during their emotes, idles and executions, simply break and stop functioning after you complete your very first match, per game bootup. They break for both the customization menu and for any other matches you do unless you reboot the game.
Nobody noticed this so far? I've seen little to no posts talking about this issue, it's so irritating to have a character do so many grunts and talk even, and during that you see a close-shut pokerface.