Blizzard has announced that the Mythic-Only Sarkareth Mount Customization has a very low chance to drop on other difficulties.
We agree with a lot of the feedback around this.The Highland Drake: Embodiment of the Hellforged dragonriding manuscript has a very low chance to drop from Sarkareth on Raid Finder, Normal, and Heroic raid difficulties.Mythic raiders will always receive two manuscripts per Sarkareth kill until the end of the expansion.