The first yields an incorrect "d:" prefix, the second the "c:" prefix. Why would this be? This is evidence of an Emacs bug.
> I don't know why you invoke Emacs from the Cygwin Bash. I suggest not
> to, or to switch to a Cygwin build of Emacs. Mixing these two > environments is asking for trouble.
When I set up Cygwin, I did not install Cygwin's Emacs installation. There is no conflict with an alternate Emacs. I am merely using Cygwin's GNU Make to execute makefiles, which correctly invoke the Windows emacs.exe binary. Why shouldn't that work?