I actually have found an intresting bug in the The Galarian Star Tournament.
After completing Crown Tundra in Sword, I copied the same save file to Shield and I loaded it in order to see the other gym leaders in The Galarian Star Tournament.
What surprised me is that I was still able to select Bea, Gordie and Klara in Pokémon Shield and play with them. It seems that the gym leaders unlocks remain stored within the save file and not the game rom.
I wonder if there is a way to use PKHeX in order to unlock every single gym leader from both games and play with them.
These screenshots are actually taken from Pokémon Shield.