Check a battle using Ditto, if it says you got boosted exp( it can only be boosted if it was traded), then the game is treating it as traded and there is nothing you can do about it except complete more trials.
If you originally owned the Ditto, it should listen to you no matter what level.
Source— using rare candies to get level 100 Pokémon before beating the first trial.
If it’s traded, there are trial requirements.
Source— Pokémon. Like any game.
3rd party software may be the best bet but I can’t suggest that here, sorry.