I'm not asking you to come up with a scenario, but just try to think about what kind of thing mother should've said that you'd find "satisfying" in this case. Some excuse? Some rational reason? Some hidden agenda?
The inverse is worse. There's no reason other than cowardice for the mom to have failed this badly. Maybe some legal reason will get made up that somehow neither parent ever communicated. All that was needed was RURIKO to stay in touch to limit damage. A realistic initial attempt would be something like "You've got our phone numbers. Remember, we'll talk Sunday nights your time when I can, and otherwise Miyako and (agent) will sort it out. I want to stay part of your life." The line and agreement isn't the point here, it's the effort and clarity that's needed to not break a kid's heart.
RURIKO didn't try, and neither parent communicated why. It was set up as a mystery, and unfair.
A satisfying narrative answer isn't that hard in concept. Why do parents who isolate child custody between them fuck up communication IRL? Embarrassment, no-contact holdover from secretly very messy divorces with or without legal fault, ambition one child is seen holding back, pride they made the right choice, narcissism that a child who is more a symbol to them than a person won't matter or will understand to let this go, shameful pessimism that it doesn't matter or their constructed hypothetical provides a happier life for the lower-talent (or a slightly higher support-needs IRL) child. There're plenty of other variations.
Pick at least two. Not difficult. Either RURIKO is a two-dimensional character with a sob story who did nothing despite apparently recognizing the harm, or RURIKO has reasons and flaws where her own baggage was unfairly placed over Yuu and harm allowed to occur and fester. With that revealed, a quick forgiveness or anything more than slow steps to reconciliation can't feel like RURIKO sees Yuu as having value in her life independent of being a musician.
I'm more hoping we get introduced to new longterm drama beyond mom and the contest, because I like the setup and characters, but the RURIKO plotline feels bungled at this point. RURIKO likes music talent, wow, such depth.