Mon-El and DawnStar are showing Supergirl around/welcoming her when she spots Brainiac 5 and mistakes him for the original Brainiac. After Mon-El says "No, wait!", both disappear from the scene entirely. Not only do they both fail to attempt to step in and clear up Supergirl's misunderstanding (other than Mon-El's one panicked feeble attempt), they are nowhere to be seen when the fight is stopped, as if they wandered off bored as soon as Supergirl attacked.