Why adding a drive letter makes a difference?

Hi,

I am mounting a VHD as C:\Windows\SoftwareDistribution folder. I then run Windows Updates. Now this works on all previous versions of Windows that supported a VHD, but on Windows 10 1703 it fails with error code 0x8024005d (WU_E_UNRECOGNIZED_VOLUME_ID). Now the funny thing is, if I assign a drive letter in addition to the first mount point it works. I was wondering why adding a drive letter makes a difference?
Any input regarding the question will be appreciated.

Thank
R

In my experience Windows Update is a strange beast and its ways are
mysterious, it isn’t necessarily written by people who understand the kernel
side of things and an assumption that all volumes must have drive letters
seems like the sort of thing that can happen. It can take umbrage at the
least things and the best to do is placate it.

R