Mega Metagross got encored and used another move.

This is kinda weird to explain so I was in Facebook and a guy said he fought against a hacker, said hacker had a Metagross non mega and used Bullet Punch to kill op's Pokémon, then op sent out Raichu and used Encore on Metagross, Metagross before getting encored mega evolved, then after mega evolving Encore connected meaning Mega Metagross has to use Bullet Punch, but op said Mega Metagross bypassed Encore and used Psychic Fangs. Does Pokémon have a mechanic were mega evolving a Pokémon before getting encored counts as a new Pokémon and the move effectively fails? Op didn't mention if it's opponent have a partner with Armor Tail or Queenly Majesty or even Psychic Terrain so there's not much context other than that and maybe there's a mechanic most people don't know. Sorry for the long post. Hopefully this explains something and doesn't confuse you guys.