So, putting together some information in case Blizzard reads this post. Prepare for a wall of text.
My main character had done all of the achievements required for the Meta Achievement “Now THIS is Dragonracing!” (Racing/Glyphs) by the morning of 3/20/24, on a havoc demon hunter. I do have the Grotto Netherwing Drake unlocked as of the Trading Post reward in July of 2023.
I had not received the Grotto Netherwing Drake: Black Scales manuscript in the mail, nor an option in the Rostrum of Transformation. I have checked the mail of every character I have, and checked the rostrum of every level 60+ character I had, it did not appear for any of them. Including using the missing mail service provided by the support page with nothing showing up missing.
Late night of 4/2/2024, I attempted to try and ‘refresh’ the meta achievement by re-doing the Dragon Racing Completionist: Bronze, Forbidden Reach Racing Completionist, Zaralek Caverns Racing Completionist, and Emerald Dream Racing Completionist, on an alt that had not done those parts of the Meta, with no success. Having checked the Mail, and Rostrum once again on alts to not having found it. The Glyph Hunter achievements seemingly cannot be replicated, as they’re account wide progress. I had also attempted doing all of the Dragon Racing Completionist: Gold, and Reverse Racer: Gold on the mentioned main to rule those out as well. Also with no success.
I have done all that I can in game on my end, including reporting the mentioned steps in game via the bug report tool.