You can try fanfiction websites like FanFiction.net or Archive of Our Own (AO3). On FanFiction.net, use the search bar and type in 'Pokemon Ash Betrayed Harem' to see if there are any relevant stories. AO3 also has a large collection of fanfictions, and their tagging system can help you locate the specific type of story you're looking for.
One possible analysis could be that in such a fanfiction, the idea of Ash being betrayed might be a way to create drama. It could involve his friends or companions in the harem turning against him for various reasons like jealousy or misinformation.
One common plot is that Ash's friends or some of the female characters in his harem turn against him due to a misunderstanding. Maybe they think he has some ulterior motives or has betrayed their trust in some way. Another plot could be a new character coming in and manipulating the situation to make it seem like Ash is the bad guy.
It could be a jealous rival trainer. This trainer might have always been envious of Ash's popularity and the loyalty of his harem. So, they plot and scheme to turn the harem against Ash by spreading lies and creating discord among them.
Perhaps a new and powerful trainer arrives in the story. This trainer manages to deceive Ash's harem members, making them think that Ash is a hindrance to their growth as trainers. As a result, they side with the new trainer and betray Ash, leaving him feeling confused and alone.
It's possible. However, when searching for it, make sure to read the summaries and reviews carefully. Some fanfictions might seem innocent at first but could contain some mild themes that might not be suitable for very young readers. Check for tags like 'All Ages' or 'General Audience' on the fanfiction platforms.
A common theme in such fanfictions could be Ash's adventures in Alola while building relationships with different characters. Maybe he forms special bonds with his Pokémon and the people around him in a more romantic or friendship - deepening way.
Well, in many such fanfictions, it could involve Ash being betrayed by Clair perhaps in a Pokémon battle strategy. Maybe Clair teams up with an unexpected rival against Ash, or she might break a promise regarding Pokémon training or a joint adventure.