I've seen it mentioned she noted having a meeting in the morning, and I believe Mio mentioned they knew the day before? I think it went down over the weekend and that's really why Mel did that stream. It's possible things were all but finalized, but there might have been some out options. As much as "NDA breeches are bad", it kind of depends what the information is and who it involves.
She knew what the likely outcome was going to be, but there might have been some negotiating going on still.
Well, Vesper/Magni seemed to start as something else and rolled over to a contract negotiation issue. Rushia is a classic case of self-inflicted disaster. But the Mel issue seemed to have happened rapidly and been ended quickly as well.
It's most likely about HoloFes. She's still a Woman and she's in the bottom tier of the company. If she wasn't doing great, it's entirely possible she was venting to someone out of the company about specifics about the selections. It's also entirely possible that Mel just decided, when presented with options since the company needed to respond after they found out, that ending her time in HoloLive made more sense. Since HoloLive isn't her primary job, it's pretty easy to see the logic loop of "I messed up, this is a lot of work and I don't feel like I'm getting out of it what I want. And I'm exhausted, so it's time to just be done." While a Rrat, that fits the information given and human nature.