Hello @Hans_N , as you’ve discovered, just because Roon knows about an artist, doesn’t mean that the artist will appear in the editing tool. The only artists that appear there are those already in your library.
Adding a new artist via creating one will result in one that is unknown in the wider sense to Roon.
To get round this, find an album that has Aki Takase as a credited artist and add that to your library. Then search for Aki in your library artists and merge them both (one way is search using magnifying glass, choose the library only icon, then select the two Akis using click/right click, or long press; a merge artists button will appear).
Once done, you can remove the added album.
In future, add tbe album first - then you will be presented with the correct artist at your editing of credits.
EDIT Here’s a helpful post.