I'd love to hear a counter-argument to the very obvious problems in this theory so we can once and for all put it to rest (and on the off chance that Matsushita is actually a traitor, Kinu will have some work to do too)
TLDR: For the Matsushita traitor theory to work, Matsushita must independently infer Ayanokoji’s plan, choose to protect it without being asked, accept serious exposure risk, and gain no clear reward.
For starters, the theory needs to provide a concrete motive strong enough to justify Matsushita taking that risk (which I will elaborate on later); without one, "Matsushita betrayed the class" explains the event mechanically, but not psychologically. Unless Matsushita has some strong personal motive - love, hatred, fear, loyalty, or something comparable - I cannot see why someone who merely signed a contract would randomly take action beyond what that agreement requires. Why should she take any risks? Even if she could accurately and confidently predict Ayanokoji's strategy in Y3V3 when she saw his GPS freeze, why would she keep it a secret at all? All she needs to do is operate on Ayanokoji's requests and get paid in private points.
Second, there's the aspect of risk. Ayanokoji's move is bound to get attention. The class wouldn’t wait long before questioning Matsushita about what she saw on the screen. From their perspective, Ayanokoji covered a humanly impossible distance and attacked them. "Did you see him on the map?" would be the very first question they all ask Matsushita. Of course, she can lie, but what about the other commanders? They all see Ayanokoji's position. If any of them or anyone who heard about it from them says something for whatever reason, Matsushita is done for. We can reasonably anticipate some form of alliance between Ryuen and Horikita in the future to counter the Kiyonami alliance. What if it forms very quickly, with the incident still fresh in their minds? What if Ryuen questions Horikita about the attack to learn more about Ayanokoji, and finds out Matsushita never reported that his GPS froze? We know the next exam forces students from different classes to be in the same "group" - isn't there a chance they would talk about it? Even putting the next exam aside, they would still all be back on the ship for a considerable amount of time. Given how unusual Ayanokoji’s move was, it is reasonable to assume it would make waves and become a topic of discussion among the third-years. The risk is just too high, which brings me back to my question - why would she take such a high risk for virtually no reward, when she was never **asked** to take that risk in the first place?
And lastly, Ayanokoji himself. He has his own goal, which is to be defeated. Kushida is a unique case here because she was supposed to be expelled and is some form of an experiment, but besides Kushida, why would he steal more talented students from Suzune? Unless they are offered a sweet deal, their only alternatives are giving up or doing their best to help the class they are in to get to A. By making an offer to Matsushita, he would be removing a perceptive asset from Horikita’s class, which seems difficult to reconcile with his broader goal of developing Suzune into someone capable of defeating him.
For this theory to be true, both Ayanokoji and Matsushita would have to act out of character, while the story would need to ignore several obvious risk factors. That makes the theory far more convoluted than convincing.