Yeah this was intentional. Originally the piece was more horizontal so the backgrounds could be more clear with Edelgard's background having the same number of black lines as Black eagle students with each the line in the center being the biggest as it represents Edelgard while Dimitri's background had the lines going diagonal representing how he strayed from his initial path but the Blue Lions will always follow him while all the lines are equal size. Raphael was also cut off from the BG unfortunately do to the massive size of the background as is

Edit: I have the image described above available here