Tribune does not equal a scroll so, if you get Tribune prior to grabbing a Favor of the Emperor tile, you'll need to get yourself a second scroll if the current tile requires one. Or you could just wait until the next turn and hope that the requirements are something you already have.
I didn't think it was possible to get a 2nd scroll once you already have a Tribune. Same with the Temporary Favor of the Gods once you already have Eternal.
I would probably rule that a Tribune still counts as a Scroll, until this gets officially clarified.