Another Redditor, u/joshhamilton235, appropriately described Vi as Powder's "emotional crutch," saying: "She was told by Vi that she was finally ready and able to do a job like she's one of the gang. When Vi told her to sit out because of her recent screw up and that she was now not ready, it was as if her guardian was abandoning her just like the rest of the gang wanted her to stay behind whenever it came to doing an important job. And thus due to her abandonment issues, she suffers a mental breakdown due to her belief that her guardian and emotional crutch had now left her behind just like Mylo and Claggor wanted. Her breakdown is a result of what happens when a kid with abandonment issues feels as if they have in fact been abandoned." Indeed, abandonment issues can manifest themselves in the exact way that Powder reacts when left behind.
For most people, having their sister leave them behind wouldn't result in them having a mental breakdown, but for Powder, it was inevitable as well as understandable. She grew up in a breeding ground for trauma, tragically lost her family, and her only remaining blood relative is, in her view, "abandoning" her because she deems her not strong enough. The reality is that there didn't need to be any abuse on Vi's part for Powder to end up the way she did — if anything, the blame thematically lies in the inequality and instability experienced in the underground. And as a result, at the end of "Arcane" Piltover must deal with the fallout of having exploited Zaun without ever helping its destitute people.
