This behavior is due to issue 37609, i.e. ntpath.splitdrive fails for "UNC" device paths.

    >>> ntpath.splitdrive('//?/UNC/server/share')
    ('//?/UNC', '/server/share')

The result should be "//?/UNC/server/share". A path on the "UNC" device requires a share component, on which is mounted a local or remote filesystem directory. It's functionally part of the 'drive' path. Using just the root path or a server path on the "UNC" device is malformed in the context of a normal file API open. The former fails as ERROR_INVALID_NAME (123), and the latter fails as ERROR_BAD_PATHNAME (161). 

The incorrect splitdrive result in turn makes ntpath.split misbehave:

    >>> ntpath.split('//?/UNC/server/share')
    ('//?/UNC/server', 'share')
    >>> ntpath.split('//?/UNC/server')
    ('//?/UNC/', 'server')

The correct result should be ('//?/UNC/server/share', '').