I’m not sure about the racial purity thing. From what I understand, they found the remains of Cleopatra’s full-blooded sister. She was bi-racial. Google “Cleopatra’s sister” for further information. There was a tv documentary from what I remember. Maybe the Ptolemys were essentially Greek at the beginning of the dynasty, but as time went on, they did mate with the locals.
What I could find indicated that at best her bi-racialism is uncertain.
I would be surprised as the break Alexander had with his Macedonian Generals was over race mixing. While at the same time human nature would indicate that racial purity is difficult to enforce.
I wonder if there were taboos against race mixing among the Ptolemys?