There's definitely something personal and it's not as simple as what she told MC, there is some other reason. We know Ella liked MC at school, did Maddie make her back off or she said she liked him and Maddie snuck in before him. Maybe it's nothing to do with MC, she thinks Maddie slept with her boyfriend. Whatever it is, it will come out, she has her reasons and she's still my #1 (so I know I'm biased).

But, like you said, there's a public photo of MC kissing a superstar and she doesn't care at all. It's only Maddie she has a problem with.
Maybe it's as simple as they were FWB and Ella is friends with the very insightful Sage:
"Girl, that friendship sailed the moment she sat on his dick. It's a hierarchy, yeah? And best friend is way below ex-girlfriend. In that hierarchy, it's either up and get promoted to a spouse or get the fuck out. You can't get demoted back to best friend once you've gotten that promotion."