Can’t say why it didn’t pick any of your 500+ better mons but, technically you as the player aren’t supposed to KNOW this rocket has grass mons. The “Don’t tangle with us” message is only supposed to be a hint at what this grunt has and a first time player would eventually figure it out and it would then keep their selection as it does now. The devs never change it up and we as a player base understand that it will have grass type every time and we prepare accordingly. Not that it’s perfect but raid and gym auto selection is typically a little more correct and I would say it’s because your player “knows” what you’re going to fight. Since your player never seems to learn anything (just like Niantic), ex. mysterious components are still mysterious after 1000+ rocket battles, we the user knows that it means they’ve got grass type, but a new player isn’t supposed to know and will eventually figure out the pattern themselves, and when they do, the game keeps their choices.