It seems that if an older dungeon is slated to be in season 1 of M+ you cannot do it on mythic mode. Tried with Siege of Boralus and said it was unavailable but I am able to get in other BFA dungeons on mythic difficulty.
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Can confirm, just tried to do the same thing, Heroic and Mythic both are unavailable.
Same issue here. I’m trying to do Siege of Boralus for the “Losing my Profession” achievement, and it’s not accessible. Very frustrating.