Commentary for Kiddushin 133:14

R. Shimon [ben Lakish] thinks that the mishnah’s general principle has exceptions. If a convert marries a mamzeret, which is allowed, the child is a mamzer. Note that the child of a mamzer/et is always a mamzer.

R. Yohanan responds that the mishnah agrees with R. Yehudah who holds that a convert may not marry a mamzeret. Therefore, this is a case of there being kiddushin (meaning they would need a divorce to separate) but with a transgression and in such cases the offspring follows the inferior, flawed, genealogical status. This is the second clause of the mishnah, not the first.
This is alluded to in the mishnah with the words “in any case.”

R. Yohanan responds that the mishnah agrees with R. Yehudah who holds that a convert may not marry a mamzeret. Therefore, this is a case of there being kiddushin (meaning they would need a divorce to separate) but with a transgression and in such cases the offspring follows the inferior, flawed, genealogical status. This is the second clause of the mishnah, not the first.
This is alluded to in the mishnah with the words “in any case.”

The mishnah could read like R. Yose—a convert can marry a mamzeret. However, the words “this is the case” would exclude this case from following the rule. Although a convert can marry a mamzeret, the offspring still follows the flawed status.

If “this is the case” means that the case in the mishnah is the only case (Israelites, Levites and priests who marry daughters of Israelites, Levites and priests) then what about a halal, a disqualified priest, who marries an Israelite woman. Here there is no transgression and the offspring is a halal.
The answer is that our mishnah follows. R. Dostai who holds that in this case the offspring is kosher, not a halal.

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.

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.

The Torah prohibits marrying an Egyptian until the third generation (this no longer applies, as the original “Egyptians” have been lost). The status of generations follows the father—so if the father is second degree and the mother is first degree, the child follows the father and is third degree.

R. Yohanan could read this law into the mishnah’s words “in any case.” And R. Dimi who disagrees and holds that the offspring is a second degree Egyptian, following the mother, he would read the words “this is the case” as excluding the case of the Egyptians. In other cases where there is kiddushin and no transgression the offspring follows the father. But not in this case.

Ravin discusses how to figure out what generation Egyptian or Edomite (both prohibited for the first three generations) is. If non-Jews marry, say a first generation Egyptian marries a non-Egyptian, we follow the male. But when they convert, the more inferior status is followed, not the male. This does not follow the rule of the mishnah where the status follows the male. [A convert may marry an Egyptian convert].
The Talmud now reads “this is the case” as excluding two non-Jews who convert.

Ravin discusses how to figure out what generation Egyptian or Edomite (both prohibited for the first three generations) is. If non-Jews marry, say a first generation Egyptian marries a non-Egyptian, we follow the male. But when they convert, the more inferior status is followed, not the male. This does not follow the rule of the mishnah where the status follows the male. [A convert may marry an Egyptian convert].
The Talmud now reads “this is the case” as excluding two non-Jews who convert.

If you say that the mishnah accords with R. Yehudah who said that a convert may not marry a mamzeret, then the mishnah works well. The words “in any case” come to include an Israelite who marries a halalah (the child follows the father and is not a halal) and the case of the second generation Egyptian who marries a first generation Egyptian. The words “this is the case” exclude the cases of Ravin and R. Dimi from above.