This openssl issue on windows is specific to MinGW/MSYS which is commonly used as part of the Git for Windows package.

The solution is to pass the -subj argument with leading // (double forward slashes) and then use \ (backslashes) to separate the key/value pairs. Like this:

-subj "//C=GB\ST=someplace\L=Provo\O=Achme\CN=${FQDN}"