So currently the new Emma/Marcus scene plays out under two conditions. Either the MC isn't dating Emma, or the MC previously triggered the Emma Cuckold scene at the party. (bugs causing the scene to fire off incorrectly notwithstanding).
I kind of wish they had some slight variations on this scene. Right now the "beta" scene as written only really makes sense as if Emma wasn't dating the MC at all. Based on their interactions at the party, an Emma who is dating the MC but cucked him did it in large part because it seemed like he was into it, and it matched her kink. In that situation, the scene would be better with her showing off to the MC what she's doing, rather than sneaking around.
To avoid having too much extra work, even a small variation like Emma telling the MC to follow her and Marcus after a few minutes would work, or having her see the MC spying on her and making eye contact and smiling. Something along those lines.