Surface Book 2 battery drain whilst off

Yes you're right. After testing out, it still drains the battery after detaching the tablet, just less severe. 

Can anyone with contacts to the engineering team help?

I am frankly dumbfounded as to how Microsoft could ever release such an evidently imperfect device. The complaints about this battery drain issue have become too numerous and widespread to be regarded as a mere inconvenience experienced by a limited amount of users. Even a 1% drain on a daily basis could be judged as unacceptable (but I would still be OK with it): when the number rises to 4, 5 or even 9-10%, you know that something has seriously gone awry.

Microsoft does not want to admit their devices are defective. I personally owned 2 surface books about 18 months apart and both were returned because of the battery drain. The issue was not resolved by Microsoft because all they care about is money. I will never purchase another Surface and suggest no one else does. At least Apple fixed a similar issue when it was brought to their attention. I bet if we all boycott Microsoft they might be forced to resolve their issue.

You are right, and I would promptly return my Surface to Microsoft, if only it weren't so essential for my academic studies. I will think twice or thrice before even considering buying another Surface device ever again in the future, though.

There are many other 2 and 1 out there like lenovo and others. Microsoft pray that people need the Surface once they get them and are forced to deal with the defective units. I also had purchased the Surface just to use for writing my dissertation  and doing research. If you are going to make a stand i would return it or they still win.