Shalom Daniel,

It's great to hear from you! I didn't find an answer to your question anywhere, but I thought of a way to answer it. Let me just remark that Rashi points out that the proof is according to Rebbi Shimon only in the second case, which is even a bit more puzzling.

In the first Mishnah cited by the Gemara as a source, the opinion of Rebbi Shimon is stated explicitly. Since it is clearly attributed to him in the text, there is no need for Rashi to emphasize that it follows l'Shitaso -- that it is in accordance with Rebbi Shimon's general view, because this is made explicit in the Mishnah itself.

In the second case, where another pair of Korbanos was designated and the first is left to graze rather than be offered as an Olah, the Mishnah cited by the Gemara does not mention Rebbi Shimon by name. Therefore, it seems that Rashi highlights this case to point out a parallel -- namely, a Chatas Tzibur that cannot be brought as an offering and yet is not offered as an Olah but left to graze until it becomes Nifsal. This appears to align with Rebbi Shimon's view, and Rashi's comment may be intended to draw attention to that implicit support, even though the attribution is not explicit in the Mishnah.

The third case is somewhat similar, though not identical. There, the discussion revolves around a Korban Asham, and in order to read this as a support for Rebbi Shimon's view, one must assume that Rebbi Shimon follows the general rule that "Kol sheb'Chatas Meisah, b'Asham Ro'eh" -- that wherever a Chatas is put to death, an Asham in the same circumstance is left to graze. This principle, which is taught as a Halachah l'Moshe mi'Sinai (see Temurah 18a), explains that while a Chatas may be left to die, an Asham may instead be redeemed and brought as an Olah. Since this reading requires an added layer (that Rebbi Shimon applies his reasoning in this broader framework) Rashi may have chosen not to present it as a direct proof of Rebbi Shimon's position. Rather, it could be seen as partial support for the conceptual structure of his opinion, provided this assumption is accepted.