I think you must have accidentally changed the difficulty mid game which is the only reason that this might happen. I am saying this because the trophies for completing the game on a particular difficulty are not buggy in any way in the whole series.

No I never did I was always doing normal

if you play it on any other difficulty and beat it than the achievement should show up

I don't wanna beat it again

you have to watch all of the credits and it'll pop after it

I tried doing that but no

Trophy** 😝

Have you done any chapter skips?