How to Set Masteries in Battlegrounds
You can use a different mastery set for each of your Battle Decks in Battlegrounds.
If there is no Mastery preset enabled, the deck will reference whatever Masteries the Summoner is using at that time for the match.
To apply your current Mastery set to a deck, tap the Apply Mastery Loadout button above the deck list. If another Mastery Loadout is already applied to a deck, you will be prompted to confirm if you want to overwrite it.