I don't know if the star in Asterion's head is for real a birthmark, but it's seen in his first trial when Androgeos tries to convince the judges, so I assume that's a birthmark. Just curious, the name Asterion means Starry, that's the reason Androgeos and MC call him "Little Star".
Now, going back your topic, I suppose his birthmark will be more important during the next builds. Now, Argos' situation in one of the endings, I think this is the punishment to let MC go further with "gods' will", nothing so rare if we're looking at other greek heroes' myths