Interesting because the Achievement text says differently last I checked? It only requires completing the fated raid on any difficulty.
The dev team right now:
Edit and the Achievement:
Complete all three Fated raids in Shadowlands Season 4
It says nothing about “on Normal difficulty or higher” so this is moving goalposts. 
Oh to add insult to injury
Defeat the following bosses on any difficulty while Nathria is a Fated raid.
emph mine.
Defeat the following bosses on any difficulty while Sanctum of Domination is a Fated raid.
Again emph mine.
Defeat the following bosses on any difficulty while Sepulcher of the First Ones is a Fated raid.
And again… emph mine.
Edit:
Blizz changed the achievements… I have no words for how boneheaded and toxic this is. Doing so feeds the most toxic elements of the community like they toss gasoline on a fire. What utter garbage game design. Whomever thought this was a good idea should be fired and banned from the industry, they have NO place near any sort of game if they don’t have a sense of fair play.