I have completed the shadow mantle set mastery around 7 times now, all within the time limit but the set does not count towards the Master of the Universe.
Note that this is a seasonal char just for information.
Sometimes it takes a while for the checkbox to be ticked off. However, the game does keep track on which ones you’ve finished. Just go on to the next set dungeon. Once you have mastered 8 of them, you will get the conquest.
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ah okey, the other 3 DH set dungeons was shown right away and i know there was a bug with the shadow mantle set a couple of years ago