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_...
What's curious to me is that Apple only uses Noto Nastaliq Urdu if Urdu is enabled in preferred languages and is higher than any other Arabic-script language. [2] Is that so on your machine?
[1] https://fonts.google.com/noto/specimen/Noto+Nastaliq+Urdu?pr... (There's a slight difference in the placement of diacritics here because of a newer font version.)
[2] https://support.apple.com/guide/mac-help/latin-fonts-correct...