you enter a hideaway as soon as you receive your Explorer Kit, you’ll be facing Pokémon that average around level 28, and their levels will increase further as you earn more Gym Badges and ultimately become the Pokémon League Champion.
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The game level-scales. Once you progress, the Pokemon will be higher-levelled.
It’s not a bug or a glitch; the game is programmed to be that way. This isn’t the first time Pokemon have been found below their evolution level— like Falkner’s Pidgeotto, or the wild level 10 Salamance in Gen 7.
A more detailed list of Pokemon and their obtainable levels in the Grand Underground can be seen here
