SciresM over on Twitter stated it uses 'cryptographically secure randomness' to determine Shiny for these Pokémon. The Crown Tundra use a brand new bit of code/module to determine if a Pokémon caught from Raids in Dynamax Adventures are Shiny. is very active in the base Sword & Shield thread but has expressed that she's already aware a new code would be required for Shiny in Dynamax Adventures so likely she's already on this or too if you were not yet aware: Click to & have provided the majority of the codes for the games.
