Warning: Contains spoilers for Pokémon Master Journeys: The Series episode 109.
Pokémon Master Journeys: The Series is setting up a chance to redeem Ash’s biggest League loss in the entire anime. With Ash now part of the Masters Eight of the World Coronation Series, he now has a shot at officially becoming the greatest Pokémon trainer in the world. Whether or not he’ll succeed is still up in the air, but at the very least, the Masters Tournament gives him a chance to fix his biggest upset.
Of all of Ash’s losses in different Pokémon Leagues, his biggest and most controversial loss was to Alain in the Kalos League. The XYZ series of the Pokémon anime had portrayed Ash as being stronger and more competent than he had ever been before, and with him advancing to the finals of the Lumiose Conference, it seemed like Ash would finally win a League tournament. As such, when Ash ended up losing to Alain and his Mega Charizard X - despite Greninja having both a type advantage and the powerful Battle Bond ability - there ended up being a fair amount of backlash. The preview for the next episode received an extreme amount of dislikes and had its comment section disabled, and while Ash did go on to win the Alola League in Sun & Moon: Ultra Legends, his loss to Alain remains a major sticking point even six years after the fact.
Ash’s loss to Alain stands out as his biggest defeat in a League, but Pokémon Master Journeys: The Series has a chance to fix that. In episode 109, it was revealed that Alain is one of the Masters Eight of the World Coronation Series, meaning that both he and Ash will be competing in the upcoming Masters Tournament. That means that Ash has a chance at a rematch with Alain, and if Ash beats him, then that could redeem his loss to Alain in the Kalos League.
The simple fact that Alain is back in the anime at all likely means that those involved have this in mind. As previously mentioned, the reaction to Ash’s Pokémon loss was so overwhelmingly negative that comments on the next episode preview had to be locked, so the people working on the anime are undoubtedly aware of how much people disliked the ending to the Kalos League. There was never a good reason to do something about it in Sun & Moon, but with Journeys being a series that revolves around the entire Pokémon world, there was finally a good reason to bring Alain back and provide a chance at redemption for Ash.
It’s unknown if Ash will end up winning the Masters Tournament, and considering how the anime usually does things, it’s likely that he’ll end up losing to Leon. Even if that does end up happening, at the very least, it’s still possible for Ash to have a rematch against Alain. If Pokémon Master Journeys: The Series managed to do that, it's likely this season of the Pokémon anime would win the love of many viewers, by making Ash's controversial loss finally pay off for the story years later.