You ran into a bug.

Alright, a small update.


It looks like we're running a process that is switching the level 90 boosts that used to be bound only to the WoW license to make them Blizzard Battle.net wide. So some of you likely had a level 90 boost from a secondary WoW license that is now available for use on any of your WoW licenses on that account.


For those currently missing boosts, the process is still running, they should show back up. Once the process completed we'll see how everything looks.
https://us.battle.net/forums/en/wow/...927616#post-10