Pokémon Brilliant Diamond and Brilliant Pearl for Nintendo Switch are remakes of the original titles. This means that they have undergone some system changes that the developers seem to believe will make them more in line with modern titles.
One such change concerns the XP sharing system, which now shares XP with all Pokémon in a group. Previously, titles relied on XP sharing items that you could give a Pokémon to boost its level without it being active in a fight. In Brilliant Diamond and Shining Pearl, there is XP sharing extended to all group members and cannot be turned off.
Unfortunately, there is nothing players can do about it, as there is no way to officially disable it in the game. While this may not seem like a big deal to some, many people out there will appreciate that the game is a little more difficult, forcing them to make smart choices about what to level up and especially about speed racing and creation theories of this nature. .
It's a shame that the option to turn it off isn't present in the game, as it would have been an easy way to placate all players, regardless of their position on the XP Share feature.