Hi Sam – I don’t have an answer for you, but can say that I have been struggling with a similar issue. My lip shapes have a bit of what you describem but are certainly different. I’ve found that I can often get a clear note when playing pp, but as I increase volume it splits into the double buzz at some point.
As I have continued practicing, I have found this happens less and less. Whilst it is still a ‘feature’ for me at present, it is certainly reducing. Whilst not an answer – you are not the only one! Hopefully someone with a bit more knowledge/experience may pipe in!
For reference, I have been working through this course for almost 18 months now.