Thank you for the heads up! That sounds like a major oversight (not to mention a huge inconvenience).
Glad to hear you were able to complete it though!
I wonder if you had had a high enough level character on the same server, would you have been able to complete it. Perhaps the quest is account-wide but only for toons on the same server/in the same faction...?
Also, even though you didn't get the invitation to battle Algalon on your main character, was the quest to battle Algalon in your quest list (on your main) at least? If so, then the quest item isn't account-wide, but the quest itself is.
Wowhead currently doesn't list A Celestial Invitation (the item and the quest) as account-wide. If anyone has tested this/run into a similar situation, some additional information would be appreciated!