L100 boosts were account wide when Legion launched so you could apply them to any sub-account on your battlenet. The L90 boosts were locked to each sub-account. If you burned a L90 boost on a sub-account, then you could not burn another L90 boost on that sub-account.
Tonight when I logged in I had 2 L90 boosts showing on all my sub-accounts where I had previously burned 4 of 6 L90 boosts. Now my accounts that did not have a L90 boost available have them available again.
I did not see any sticky's in the Wow forums about this. Did anyone else see this change today?
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