I have the same problem - it looks like the flap don't open at all: warm engine / hight rpm / sport or sport+ mode... the left exhaust outlet is clean like new..
Today I talk to my BMW service partner - they say that the exhaust flap don't have a separate fuse.
Actually I can't really belive that so I will go next week to the service partner to check the car..
If you have any news, please let me/us know