I’m experiencing the same issue. I just spent a crap ton of time unlocking the Rank 3 Ripple in Space Essence and now I am unable to buy it on my alts!
Edit: The workaround above worked. An easy Rank 3 Essence to get is the Crucible of Flame from the Investigating the Highlands quest from Kalecgos in the Chamber of the Heart. Your Heart of Azeroth needs to be level 60 to unlock it.