TT #224 · Parshas Tazria · Aliyah 7 · Passuk 40
English: If a man’s head becomes bald, he is bald; he is pure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:40
Torah Temimah cites Sifra (Toras Kohanim) for the rule for a netek when the whole head later becomes bald.
Sifra — Vayikra 13, Midrash Sefer
English: If a netek the size of a gris was present and the entire head later became bald, the verse teaches that the person is clean under the netek rule.
Complete baldness changes the diagnostic framework for a former netek.
Once the entire head becomes bald, the Sifra applies the bald-head rule and the earlier netek no longer makes the person impure under this rule.