TT #206 · Parshas Tazria · Aliyah 5 · Passuk 33
English: he shall shave himself, but he shall not shave the netek; and the priest shall isolate the netek for a second seven-day period.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:33
Torah Temimah cites Sifra (Toras Kohanim) for the prohibition on shaving an uncertain netek despite another haircut obligation.
Sifra — Vayikra 13, Midrash Sefer
English: Even when a nazirite is obligated to shave as part of his vow, that shaving does not override the prohibition against shaving an area that may be a netek.
Textual derivation
A nazirite’s prescribed shaving does not permit him to remove the hair of a suspected netek; the possible sign of impurity must be preserved until the priest clarifies its status.