The problem is not really that the xp is not the same for all but rather that everyone should only get 2400xp. It Makes the First part of the game way too easy especially when higher level also means better gear available at the shops while, with a tiny bit of game knowledge, you can enter Fort Joy and escape through the sewers without a single fight right from the start (after you see Atusa'scene and are done recruiting you party members) to get your big boost. I think the difficulty in tactician was well balanced as it was, but with one extra level so early it is now pretty Casualized. Obviously, it is a bug and was not intended as such.
