SIL's Nastaliq font uses their own Graphite engine, which is included in Firefox but not other browsers (Demo page: https://graphite.sil.org/graphite_fontdemo ), but e.g. Noto Nastaliq Urdu also exists https://fonts.google.com/noto/specimen/Noto+Nastaliq+Urdu?pr... and does a decent job in non-Graphite engines, certainly better than Awami Nastaliq without Graphite.

So the real question is why Android doesn't make it easy to put Noto Nastaliq Urdu in the font stack.