Hello I will try to help you with this. I will check for service bulletins for your car and then report back in a few minutes.
BMW Master Tech
Hello. My check shows that there are some bulletins for transmission issues that match your complaint.
The service bulletin states the customer may complain of a harsh 3-2 2-1 downshift with "clunk noise" when coming form a stop. Which fits perfectly for what you have been experiencing.
Now I must tell you that the problem could be internal in the transmission BUT, the repair for the bulletin is simple. It states to have your EGS (electronic transmission control) programmed with the lates software by BMW. Your dealer has the latest software. The only cost will be for labor which should not exceed 1 hour.
Most likely this will solve your problem. If it does not then further diagnosis will be needed and the fluid will need to be checked if it smells or looks burned. If this is the case then it almost always is a transmissin replacement. But since you do not have any failsafe message most likely it is only a programming!
I hope this is helpful. Please leave me a short comment on the feedback section if you feel I have answered adequately.
Thank you.
BMW Mechanic
Level 1 BMW Master Certification, 9 Years Dealer experience, Team Leader, Shop Foreman