The sages of blessed memory are always precise about distinguishing between masculine and feminine in grammar.

When it is written פחתת היא (it is a fret)⁴⁰² in feminine, it is implied that it cannot refer to the plague.

footnotes: ⁴⁰² Leviticus 13:55.

And when it changes gender and says ולא ישאו עליו חטא (lest they bear sin for it)⁴⁰³, it should have written עליה (so it can refer to the noun משמרתי). This is so that one will not err and think that it refers to a carcass or food ritually unfit for eating, which comes in the previous sentence. And also it comes in feminine language for the purpose of a homiletical interpretation.

footnotes: ⁴⁰³ Leviticus 22:9.

Leviticus 13:55 says,וראה הכהן אחרי הכבס את הנגע והנה לא הפך הנגע את עינו והנגע לא פשה טמא הוא באש תשרפנו פחתת הוא בקרחתו או בגבחתו(And the priest shall look, after that the plague is washed; and, behold, if the plague have not changed its colour, and the plague be not spread, it is unclean; you shall burn it in the fire; it is a fret, whether the bareness be within or without), using the wordsפחתת היאfor “it is a fret”. Sinceפחתת היאis in feminine, it cannot refer toהנגע*(the plague), which is in masculine⁴⁰⁴.*

Leviticus 22:9 says,ושמרו את משמרתי ולא ישאו עליו חטא ומתו בו כי יחללהו אני יהוה מקדשם(They shall therefore keep My charge, lest they bear sin for it, and die therein, if they profane it: I am the LORD who sanctify them), using the words ולא ישאו עליו חטא for “lest they bear sin for it”. The word משמרתי (My charge) in feminine alludes to the Sanctuary, which comes in masculine. Since the pronoun of עליו comes in masculine, it cannot refer to a carcass or food ritually unfit for eating, which is in the previous sentence, since they come in feminine. Therefore, it must be referring to the Sanctuary⁴⁰⁵.

footnotes: ⁴⁰⁴ Malbim, HaTorah v’HaMitzvah Tazria 172. ⁴⁰⁵ Ibn Ezra Commentary on Leviticus 22:9.