It's really strange and I can't explain it but I had this issue until I installed the Dinan midpipe. Doesn't make any sense since I kept the stock muffler with the valve. So here's your solution, sounds amazing too
Oh and the valve is not what is enabling / disabling the pops and burbles... its the riding mode switch. So you can have a full exhaust and it will still be more quiet in normal mode vs sport mode.