I have the same problem. except, I made a Corsola Galar inherited set, and it registers as Gengar-Gmax, I even tried changing ability to weak armor instead, same effect, here is the set:
Tonk (Toxapex) @ Black Sludge
Ability: Cursed Body
EVs: 252 HP / 4 Def / 32 SpA / 220 SpD...