It's also possible that it was Snape, in a more complex scenario. Although he is the potions master, he might have unknowingly set up a situation where Harry collapses. For instance, if he assigned a particularly difficult potion that was too much for Harry's current abilities, and due to Snape's strict and unhelpful teaching style, Harry couldn't ask for proper guidance and eventually collapsed from the stress and difficulty of the task.
It could be Malfoy. He often has a vendetta against Harry and might have found a way to make him collapse during potions class, either by tampering with his ingredients or distracting him in a way that leads to a mistake in the potion - making process.
A new, mysterious character could be responsible. Maybe there's a new student or a hidden enemy in the school who has some ulterior motive against Harry and decides to target him during potions. This character could have used a very subtle form of magic that's hard to detect, like a charm that slowly affects Harry's strength as he works on the potion.
Chapter 827, "Mystical Event Investigation Bureau", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Li's words indeed make sense.
Chapter 787, "Changing Fortune", is the clearest early answer because it turns Xiao Xue's situation into a concrete plot event. The setup is How could she know about these things?, but the pressure keeps building through Xiao Xue had grown up with Yafei and knew a lot about her, but she....
Chapter 850, "Wanted to Recognize Her as a God-Granddaughter", is the clearest early answer because it turns Wanted's situation into a concrete plot event. The chapter turns that into something concrete by moving from Though Jiang Li might guess her family situation, he would never know...
Chapter 859, "I Bet She Won't Last Ten Minutes!", is the clearest early answer because it turns Jiang Bufan's situation into a concrete plot event. The chapter turns that into something concrete by moving from All morning, the two little ones were quiet, not making a peep, even...
Chapter 900, "Let You Hold Your Great-Grandchild Sooner!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event. It opens with The nerve-wracking two days quickly passed.
Chapter 630, "The Secret Manual Passed Down to Sons, Not Daughters", is the clearest early answer because the hidden truth stops feeling like mere subtext.
Chapter 693, "Is a Man's Prime That Short?", is the clearest early answer because it turns Sister's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Zhao Lang had never anticipated that the bravado he had shown earlier...
Chapter 530, "I Only Drink 80 Degree Tea!", is the clearest early answer because it turns Hua Country's situation into a concrete plot event. Early on, Old Sir Gu took a step back, his expression cold and guarded as he....
Chapter 683, "This is where you will be filming from now on!", is the clearest early answer because it turns Alright's situation into a concrete plot event.
If you want the clearest chapter reference, go to Chapter 13. Early on, Sensing that something was off with Jiang Yan, Jiang Yan dismissed the...; later, the chapter leans on What on earth do you mean?" Ignoring him once again, Jiang Li shifted her....