merlin trials glitch

[optimus_prime]

i have done all but one merlin trial in the game last one is somewhere in hogwarts valley, ive revelio on broom and everything but still cant find it any ideas how to find it?

[<MasterThomy>]

If anyone will be looking in future, I've found one east or Arn merchant which was not marked on map (assuming it's always same one bugged)...there's were few others elsewhere like this too