Why is the Arabic font on my iPhone different from what it is supposed to be?
It seems I had my language preferences list with Urdu higher up than Arabic, so it used the Urdu font (Nastaleeq) wherever it saw Arabic text. Putting Arabic higher up in the list fixed the issue and made the font San Francisco.

As a side note, it is kind of a shame that it can't distinguish between Arabic and Urdu (seeing as they use the same alphabet), and yet it assigns a separate font to each of them and uses said font depending on its position in the language preference list.