Hi and welcome. I don't believe it is covered under warranty especially of it's a AUC warranty. Basically the design of the rear lights was flawed and BMW changed this later on but you have to pay for faulty lights to be replaced, I believe it's around £300. You could always try and get some goodwill from BMW. Eventually it will manifest itself into causing the lights to fail and a warning will appear on the dash. Is it both rear lights or just the one? Any I have seen have always been the offside (drivers side) light only