This game, and others like it, aren’t made to be so specifically attuned to canon. If they were, we wouldn’t have it to begin with because technically we wouldn’t be able to get close to the majority of these dragons.
From what I can tell so far, the team has done a very good job in doing the best they can in keeping to canon yet still having to make some ‘adjustments’. Tweaks and slight differences so that we have a game at all. And there are still going to be purists that can’t stand the idea of the dragons fighting and won’t touch the game.
It’s not a bad thing, it’s just how they prefer to take in their franchise/fandom.
So there are going to be certain, basic, aspects of a mobile game that are going to make their way in. Things that sell, things that draw in more consumers/ a bigger playerbase. An Arena is definitely one of them.
Technically you don’t actually have to participate in the Arena. Just ignore it. It doesn’t pop up, that I’m aware, unless you go out of your way to look into the tab. You don’t lose fish or coins or scales- just another opportunity to gain them.
Other players might defeat your team while you’re not looking but it shouldn’t negatively affect you if you’re not actively participating in the arena. So you’ll be golden.
Still, from how you worded it, it sounds like you DO get notifications? Like…push notifications? Because I don’t. But I think I turned a few off in my in-game settings. I’ll check to see if that was an option implemented.
EDIT: No, I have all notifications enabled. And I don’t remember getting any for arena defeats. Not to say this isn’t just a bug, but you might be able to just turn this off.