> If I am indeed set up this way (each mailbox is fetched independently AND each independent mailbox is set to always go to a certain queue),
why didn’t the ticket get created in all of the queues to which it was sent?

Good question.

1) Did all mailboxes receive a copy?

2) Are there any indications in OTRS that the message was discarded or copied multiple times to one ticket?

Otherwise, probably OTRS is preventing duplicates based upon message ID and if so, not much to do about it. Making three tickets isn't generally an optimal solution because three different people working on the same ticket is considered redundant labor. Normally, a ticket that comes in to the main Queue would notify everyone who had that Notifications Yes and the Queue highlighted in the user's My Queues.