Author here.

Well of course Spring + Japan = Sakura. In a way the model would be faulty if it didn't reach for that cliché. That's what it's meant to do.

If you want originality, you need to supply it. AI fills the vacuum with the obvious thing.

And in a way that's a feature. If I was creating a poster myself I'd think "nah, can't use sakura it's too obvious". AI gets you past that hurdle. "Yeah, we used the cliché, it looks nice, let's move on".

This is if it's for a school fayre, village fête, etc. - brilliant originality isn't the goal. But it's nice when your poster doesn't look the same as three other posters on the same noticeboard (I have seen this).

If brilliant originality is needed, a human has to bring it. Although an LLM with the temperature turned up can offer some nice seeds.