Unable to Purchase Rank 4 of Mythic+ Essence from Thaumaturge Vashreen after Completing the "Keystone Master" Achievement
- I've completed a Mythic+ 15 and Thaumaturge Vashreen will still not sell me the essence.
- I have the Keystone Master achievement but the essence is still locked.
As the tooltip specifies, this requires not just the base Battle for Azeroth Keystone Master, but the Season 3 (or later) version of the achievement. Battle for Azeroth Keystone Master: Season Three.