Totally reasonable and realistic situation - I personally went through something similar myself with my break up after 7 years. It wasn't a big reason, or a fight, just different priorities, spending less time together, not really feeling the romance aspect of the relationship as much as friendship and companionship, but at some point someone realizes they want more and it's time to move on.
I imagine we'll have a time skip at some point where Sui becomes an adult, or we'll just have some other plot entirely, as I didn't expect a break up all of a sudden, even if it made sense. Good series so far, surprisingly.