Infinite Swain Ult With Manaflow Band

Just finished up an aram and noticed something that is absolutely insane. Now, I don't know if it's really fair to call it a bug, maybe it's supposed to work like this, but if not, something should really be done about it. It seems that when Swain is using "Manaflow Band" his ult doesn't reduce his mana when the proc is up (as in, when the next ability cast wouldn't cost mana and instead regenerate it). I was being chased by a swain and he just wasn't losing mana. His mana would "drop" by however much it's supposed to, but then he would instantly regain it immediately after losing it. It was like if you were playing him in URF. At the time he did this the Swain had full mana. I don't know if the bug only happens under that condition or what but this should probably be looked into. Because, while I could be wrong, I'm guessing Riot didn't intend for Swain's ult to be permanent outside of URF.