>>46107908>>46110185I just replayed Sun myself, and OP is right. She was crazy prior to the events of SM and that's the reason why Gladion and eventually Lillie escaped. When you visit the Aether Paradise for the first time and fight the Nihilego, that's the first time she saw one and became obsessed with them there, but you can already see something wrong with her. She doesn't actually get affected by the toxins until she merges with the Nihilego and goes turbo cray-cray. Mohn's notes do mention them but that implies that she became obsessed with the idea from his notes after he disappeared, which is what made her crazy. The dialogue itself is also specific that it was the events in Ultra Space that causes this level of damage to her, to the point where she was bedridden.
Like almost everything else, USUM ended up sullying this by having her just abandon her crazy personality after the encounter with Necrozma. Seeing as she wasn't chasing Nihilego after that it seemed that it was the true cause of her original mental state, but really she remains as a ticking time bomb of BPD.