I jjust got Smeargle to photo bomb me. According to one of the Pokémon groups I belong to, it seems like it happens with the first photo you take of a Pokémon if it’s going to. So switch Pokémon and take photos of dozens of Pokémon instead of dozens of the same one. Also it seems to only happen with AR and not AR plus. This could be wrong but eventually it worked for me.