This led to a natural question: what bonus would there be if you used Promotion Items for that character's affinity to level up? Sadly, the answer is that there would be no extra bonus like there is with Training Items, only the typical affinity bonus.
Update courtesy of Chowbacca: Using same-affinity Promotion Items provides the same XP levels as Training Items, but there are 2 disadvantages to this.
Screenshot by Chowbacca
The first is that character-specific promotion items are harder to get than training items because promotion items are more random. The second disadvantage is that there is that the Tier 1 promotion item does not have affinities associated with it, so you don't get to max out on the "straight flush" method like you would for training items.
Have you found any combinations that yield special bonuses besides these? Throw them in the comments! Enjoy the game and thanks for reading!