I recently read a post on a discord that someone reported that the level 58 boost is 1 per Battlenet (Bnet) instead of 1 per wow account on a Bnet. If I recall correctly the poster had 8 wow accounts on their Bnet and after buying the 58 boost on 1 wow account they could not on any others on their wow account.

Please chime in if you encounter this or not.