I just noticed the pocket morty's sorta changed the start of the game when you catch morty isn't the original one it was "Scruffy Morty" and instead it was replaced by "Baby Morty" during the run of the game.
god, I remember playing this game on and off as a kid, but the only reason I stopped completely was because somehow all my data was deleted and there was no way to recover it, so while going through the stages of grief from losing everything in the game, I uninstalled it and never looked back.
I bet the reason you weren't progressing the defeat trainers achievement is because you reseted your save file so the counter went down to 0 internally so you would only progress if you reached the same amount of progress as before
Pocket morty was genuinely one of my fav childhood games id always play it after comming back from school i used to love it sucks that they took it down:(
once came across this on the play store and i was like "why is there a pokemon knockoff based on an adult swim show?" and only played a little. That poor soul of mine back then failed to realize how different it actually is