BROO, MINE DID IT. I'm not a hundred percent sure but this is why I think she did it: She wants everyone to mistrust Isanuma so then Mine can console Inasuma and make her feel like Mine is the only one that truly understands her/believes her. (Also, at the end of the chapter, it shows some stains on her shoes, most likely the paint.)
I genuinely don't know how I ended up here but I'm not complaining 😼
YURI!YURI!YURI!
YURI!YURI!YURI!