Boss, I'm really sorry to hear that but personally, I'm not very sure what might have caused this either (technically). What I can offer you however is some advice on how you could proceed this with AB. I believe you need to be a bit more stern and assertive.
1) Tell them that their cars are not cheap and that you have spent so much to own a BMW, you expect top notch customer service.
2) It is disappointing that they do not show enough responsibility of the issue and you deserve to get better.
3) Push them to get the issue fixed by hook or by crook. If BMW does not know how to fix this, who else would?
4) Ask for the next course of action and what's the ETA for it's investigation/diagnosis. What is BMW's effort to identify this issue and how to fix it.
5) Continuously pester them until they get this issue fixed/resolved. Tell them that by saying "it's normal" or "i don't know" to you is an unacceptable answer.
6) Ask for an escalation path and who you can escalate this matter to if it is still not resolved.
