Actually I think it may be related to the latest update since it unlocked for me the day I installed it.

Hi, I have just noticed that the achivement actually unlocked for me couple of days ago! I don't know if somebody fixed the issue or if it just decided to unlock on its own, but it is a nice christmas present I guess. (note that I have done nothing mastery challenge related since posting this topic - I didn't play any of the challenges, I haven't talked to Hildiran)

@ubi-froggard

@ubi-froggard Hi, sorry for the late response. The trophy is called "Mastery Challenge" and it's from AC Valhalla

I have recently noticed that the core trophy for mastery challenges didn't unlock even though I have completed the storyline a long time ago. Can this be fixed somehow?