This is a problem related to WebDAV and nothing we can directly influence. This is just the way, Microsoft wants it to be:
"Research on WebDAV protocol RFCs and verification from Microsoft's product team confirmed WebDAV protocol doesn’t support querying server’s disk capacity" - Microsoft
Microsoft should fire their research and product teams, as apparently they haven't heard of RFC 4331 and thus show the capacity of the system drive.
We currently implement an alternative way of providing the virtual drive. As soon as this is implemented this problem will be solved. The progress is documented on GitHub.