The rule is about an established feminine alif, not merely a similar-looking ending. Compare صُغْرَىٰ with فَتًى and مُصْطَفَىٰ: all end in a maqsurah shape, but the latter two are masculine. Shape alone does not prove feminine status.
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Do Not Judge Every Final Alif Alike
Cause of Ghayr Munsarif: Feminine Alif and Feminine Proper Names
The rule is about an established feminine alif, not merely a similar-looking ending. Compare صُغْرَىٰ with فَتًى and مُصْطَفَىٰ: all end in a maqsurah shape, but the latter two are masculine. Shape alone does not prove feminine status.