is this a manga thing, or is Jpn weird about this?
both.
Japan is weird about the "purity" of its celebrities, especially idols and young actresses, as they heavily market and push the parasocial aspect of fandom and basically encourage lonely, desperate fans to obsess over them and imagine that they have a special relationship with them not tainted by the idea that the famous person could ever actually do something so selfish as love someone else because they "belong to" the fans.
Like, in the west you get gossip like "<famous actor/singer> was seen in public with someone. Who is it? are they in a relationship?" whereas in Japan it's like "<famous actor/singer> was seen in public with someone. How dare they betray their fans like this." It's most prevalent with idols since that purity is like a core feature of their marketing and the expectation is that most of them will "age out" of being idols around the time they get to marriageable age, at which point it's kind of expected that they'll have a big farewell and then retire from the industry before tying the knot, whereas regular actors and actresses and other famous people can go public about their relationships and continue their jobs and it doesn't matter as much.
That said manga will hugely dramatically overplay it for story purposes. Without pictures of the two of them or it potentially being something salacious like they're coming out of a hotel or whatever it seems like this would be the sort of thing that would just blow over because it's a big load of nothing. But in manga-world it's going to have her manager come down hard on both of them for making her life difficult and it'll probably lead to renewed interest from that actor guy who will try and badger her into a relationship with him because they're a better fit for each other since people like celebrity couples and it'll probably be the engine that keeps the two leads here from attempting to recognize their feelings and get together for another like 38 chapters until we get to the closing pages of the series.