The filter is there to instantly eliminate toxins from water (somewhat miraculously if I do say so myself). Pepsi Max has so many toxins in it that several were probably filtered out, leaving you with whatever was left over. Makes sense that it would taste funny.
It was probably just the Pepsi Max that tasted unusual. Either that, or you need to wash the filter! =)
I honestly just laughed out loud at this post. Love it!