Why do you think someone who sees you regularly would bring up important topics over the phone or LINE instead of talking about them in person???

Maybe you just happened to think of bringing it up then and decided to contact them right away…

There could be all sorts of reasons, and it probably depends on the person, so I can’t say for sure. Personally, I don’t like being asked when I’m available without being told what it’s about, so when I arrange to meet someone, I explain the purpose. Even if we already have plans to meet again, I think it’s better to bring up anything important sooner rather than later. Information is most valuable when it’s fresh, and I can’t really imagine an important matter that can wait indefinitely.