Actually you have to set it above 1 to see some changes otherwise anything multiply 1 is the same.
I don't think this is an XP Multiplier issue. I think this keeps happening (and it keeps happening to me too) because the option of 'Player Level' is always 1. I've noticed that it never increases with Jokers level and just drops him back down to level 1.
Actually i only got 1 issue with it (dropping to lvl1),it was right when you do the Emperor 1.
For some reasons ,after the fight ,Joker lvl would drop always to 1 even if i didn't use any cheat (except editing money way before that point).
After i closed the trainer i didn't had any problem with that particular story point but now i'm a bit wary cause at first i didn't notice,i thought it was just a visual bug going from lvl 1 to 5 at that point in game,plus,during the fight ,the HP/SP were still ok....