وَلَا الطَّاهِرَةُ خَلْفَ الْمُسْتَحَاضَةِ،
Meaning: A non-excused, ritually pure woman does not pray behind a woman with legally excusing istiḥāḍah.
Istiḥāḍah is bleeding outside the legal category of menstruation or postnatal bleeding. When the conditions for a continuing legal excuse are met, the woman’s prayer follows the relevant concessions. The issue concerns differences in ritual purity, not merely gender.