مُسْلِمَاتٌ: nominative with ḍamma and tanwīn.
رَأَيْتُ مُسْلِمَاتٍ: accusative marked by kasra.
مَرَرْتُ بِمُسْلِمَاتٍ: genitive marked by kasra.
That is a sound feminine plural, not the ultimate broken-plural pattern. Do not choose an accusative ending from plurality alone.
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Diptote Causes — Ultimate Broken-Plural Patterns
مُسْلِمَاتٌ: nominative with ḍamma and tanwīn.
رَأَيْتُ مُسْلِمَاتٍ: accusative marked by kasra.
مَرَرْتُ بِمُسْلِمَاتٍ: genitive marked by kasra.
That is a sound feminine plural, not the ultimate broken-plural pattern. Do not choose an accusative ending from plurality alone.