From #comment:10, ICU is seeing a failure due to missing U+0650 ARABIC KASRA missing from Sindhi exemplars. By looking at online sources, such as https://sd.wikipedia.org/wiki/%D8%B3%D9%86%DA%8C%D9%8A_%D9%BB%D9%88%D9%84%D9%8A, I can confirm that fatha, damma, and kasra are used in Sindhi.
Since it's not clear if they're a required part of the orthography or not, I believe we should add these three to the Sindhi exemplar as auxiliary characters:
U+064E ARABIC FATHA
U+064F ARABIC DAMMA
U+0650 ARABIC KASRA