«وَكَانَتْ مِنَ الْقَانِتِينَ»
And she was among the devoutly obedient.
The feminine kanat and masculine al-qanitin are not incompatible. They participate in different syntactic relations: the verb agrees with a female subject, while the plural names a broader class.
One must identify what each form refers to before judging agreement. The sentence is not governed by a single gender marker throughout.