This episode... works.
I'm not going as far as to say "Shudo is rolling on his grave" over this one. Unlike what other people might think, I do believe that those "auther" works wether it's the first Pokémon movie or even Watchman can be "touched" and there isn't much of an issue giving them a follow-up. At the end of the day, they are still work from hire done by people who needed some bucks to pay their bills (the only difference is that Shudo was smarter than Moore on keeping his rights, but I digress)
That said, while I'm not opposed to this episode existing in the first place, I'm somewhat dissapointed because... It just exists. They try to make a big deal out of it but... I just don't feel it. They claimed it was an episode meant for the "parents of the kids watching who've seen the movie back when it originally aired". Well, while I'm not a parent, I certainly was a kid back then and I don't feel that episode is truly "for me". Honestly, I just don't understand what's supposed to be "for me" besides revisiting Mewtwo? Ash is there. So is Pikachu but then they've been around forever.
It also helps the episode is completly disconnected to the movie. There's no thing truly connecting them aside from Ash knowing Mewtwo (and here the writting deserves some credit as they really nailed how to show us Ash and Mewtwo know each other without saying). I mean, at least you could somewhat tell us the island Ash and Goh are coming is the same one we've seen on movie 1. I mean, for all we know this island might be, but some confirmation would be nice.
Another thing is that the update on Mewtwo is... Dissapointing. Not to mention it kind of doesn't match the end of Returns. Some of you won't remember but back on Gen 5, there was speculation Mewtwo was on Unova because the setting of where he was in the end of Returns could pass as Castelia or something like that.
While it's logical Mewtwo is babysitting pokémon mistreated by humans... It's nothing quite new. I mean, game lore says the muskedeers does it. We've seen a story about it with tsundere Volcanion on movie 19. I dunno, if you are making a big deal about Mewtwo returning then think up something that makes it stand out like Mewtwo is also saving/helping humans or something like that. I dunno, anything that shows some progress.
I've seen some criticisms towards journeys because it tends to pull off some things mostly to say "look at me, I'm here!" or some cheap nostalgia baits (the Piplup and Croagnuk episode). It wasn't really bothering me but alongside with the "Ash's Lucario" thing it feels more of a marketing stunt than something that benefit the narrative.
The episode also keeps a trend of Journeys of pulling a Brian Bendis and just repeating the same things over and over again. I mean, why do I need to see Ash and Goh reaffirming their dreams? Gee, that thing is mentioned in the beggining of every episode through the narration. And while it's true they probably did that for the sake of people who might be unfamiliar with it, I dunno... It just feels borderline repetitive.
It also doesn't help there's no justification/whatsover for why they need to have the boys reaffirming their dreams to Mewtwo nor he needing to test their resolution or come up with something like "this world is too big, don't think things will be as easy". Why do they even need such lesson?
I mean, why do we need Mewtwo returning to give a lesson to Ash and Goh about to not get overconfident when we saw no signs of them needing to learn anything, specially something they already knew. Gee, Goh's remarked that the world's is big/vast as soon as episode 4! Also, Ash learned there's always someone better as soon as his first match with Bea.
Gee, if it came right after an episode or scene showing that Ash and Goh got overconfident after getting their brand new evolutions, the pairing with Galar doggos and the Eternatus catch, it'd work a lot more in a "don't think things will go your way like with the Darkest Day incident"
This episode really feels more like made out for the sake of fanservice than anything else. But even at that it fails.
Honestly, if it wasn't for the branding of using Movie1!Mewtwo, this episode could be just like any other Journeys filler. Gee, I'd say if it was any other legendary, it might have worked a bit better because at least they'd hype it slightly less and worst case, I'd be complaining about once again the episode spelling out Ash's and Goh's goals.
Not wanting to sound grumpy, but I dunno, the episode doesn't live up the hype they were making for it. Lucario's and Cinderace's performance was on par with your standard battle (though I'd say Raboot vs Flygon was more appealing).
Either way, the episode is functional to what it sets to be thus why I think it works. But I dunno, for such a hyped episode, it could deliver a bit more.