• 6.11.1:
• Window 10 x64:

In windows, path.join('C:') return 'C:.' not 'C:'
In other system, such as linux,macos always return 'C:'

In windows, 'path.join('C:\')' is corrrect, return 'C:\'

But you give path.join more than one args, it's correct.
such as path.join('C:', 'a') == 'C:\a'

here is a interesting thing, if you run path.format(path.parse(path.join('C:'))) in windows,
returns 'C:C:.'

I don't know why...