In some discussion about Arabic rendering on another website[0], it was pointed out that the Basmala is its own codepoint in part because it is (or was?) a legal requirement on Pakistani documents and comes from an Urdu character-set. It's possible that, as a character effectively originating from and used by Urdu speakers, Apple defaults it to Nastaliq regardless of your font settings.
0: https://lobste.rs/s/7s4sjp/u_fdfd_arabic_ligature_bismillah_...