Just because we don't see it, doesn't mean it didn't happen. It easily could have happened in one of the periods where there is a huge jump in Days and we wouldn't know. It isn't necessarily a plot hole, it just wasn't shown.
Lack of mention =/= plot hole.
Hm. Maybe I should make this clear. The novels are something called secondary canon, while the games and Nomura Interviews are primary canon. In the event that there is a contradiction between the two, the primary canon wins.
It isn't a true plot hole, because the novels aren't on the same level of canon as the games.
I know this is probably a bad example since not many people watch LOST but... here it goes. For LOST, the primary canon is the show. The secondary canon is things like videos at Comic Con. More often than not, the stuff at Comic Con is canon and is worked into the show, but sometimes they can't get that content into the actual show (an example would be the Pierre Chang video Daniel filmed). In that event, the primary canon wins in terms of the mythology of the show. Where as the secondary canon is used more as a way to promote the show. Much like the novels are a way to promote the games.
The fact is, the novels aren't written by Nomura. What happens is an author goes to Nomura and says they want to write __________ storyline. Nomura then okays it, or tells them to change it or whatever. It's loosely canon, but since it is another medium writing it, they tend to throw in scenes and dialogue that are only there for "fun" and what not. It's an iffy subject, but really, the only canon you can have 100% faith in is the primary canon, not the secondary.
too long. v.v