Commentary for Kiddushin 133:6

If a halalah (a woman disqualified from marrying a priest, for instance a divorcee who had relations with a priest) marries an Israelite, the child follows a male. This is another example that would fit into the first clause of the mishnah and yet it is not taught.
The answer is that the words “in any case” in the first clause are there to include this case.
This case is not included in the mishnah because it would not easily fit in. While a halalah can marry an Israelite or Levite, she cannot marry a priest.