Mel is REALLY stupid and kind girl. Having massive laps of judgements because she just assumes good things, instead of using the brain and realizing it's very bad is on brand for her. The most likely scenario is she blubbed something to Kson, thinking it's not a big deal and she will never talk about it, because it would be trouble for Mel and they are good friends, even knowing full well she shouldn't talk about it. Like, "oh, we can't meet up on this date because we will have a concert on that day" level of innocent, but before it was announced and therefore leaking the info. Then Cover found out about it, or even she herself mentioned it, and we have what we have.
The reason given combined with what seems to be a mutual? termination of contract is making me think it is something like you state, but something she keeps doing over several years and won't stop despite being told not to over and over, putting Cover in the position of finally coming to terms that she is just too stupid and there is no way to keep her from making these mistakes.
This would be the kindest, most understandable, and most amusing reason for her termination.