Historical revisionism?
"The problem was posed between the third and fourth centuries, when a Church that had become widespread and well structured made the great and brilliant wager that is at the basis of our entire artistic history. It accepted and made its own the world of images, and accepted it in the forms in which the Greco-Roman stylistic and iconographic traditions had developed it. It was in this way is that Christ the Good Shepherd took on the appearance of Pheobus Apollo or Orpheus, and that Daniel in the lion's den had the appearance of Hercules, the victorious nude athlete."But how could one represent Peter and Paul, the princes of the apostles, the pillars of the Church, the foundations of the hierarchy and doctrine? Someone got a good idea. He gave the first apostles the appearance of the first philosophers. So Paul, bald, bearded, with the serious and focused air of the intellectual, had the appearance of Plato or perhaps of Plotinus, while that of Aristotle was given to the pragmatic and worldly Peter, who has the task of guiding the professing and militant Church through the snares of the world."
I would gueaas that if the artist had drawn him differently, then he probably wouldn’t have gotten paid or any future commisions.
Even the Roman portrait busts, with their sometimes brutally frank depictions of brutal thugs, bureacratic timeservers, and epicene emperors, have some aspects of symbolic portraiture. Sometimes it was sly criticism (as when Commodus was portrayed as Heracles, looking like a little kid playing dress-up) but sometimes it was aspirational. The family portraits from Herculaneum of the Balbus family are a good example.
Certainly an artist would adopt the symbolic language of his trade in making a posthumous portrait. And Paul as Plato and Peter as Aristotle makes perfect sense, if that's what you're working with.