At the lake of Outrage, in the Pokémon Shield wild area, I encountered a wild Chandelure with WANDERING SPIRIT, which isn't supposed to be one of its abilities. I have never glitched or hacked the game, have never bought Pokémon online, etc. What happened?
Most likely YOUR Pokémon had Wandering Spirit, it got swapped to Chandelure when it hit you, and you mistook it for originally having that ability. If you didn't have Galarian Yamask/Runerigus, your game must've screwed up somewhere.
Hope I helped