As reported by Ramses Rodriguez Martinez in Debian BTS:
mount.cifs manpage states the credential file format is:
however, it should be:
....From what I see in mount.cifs.c, our user is right..:-)
You are perfectly right!
Fixed with commit 3b7f8a759 in master and pushed to v3-4-test and v3-3-test.
Closing out bug report.
Thanks for reporting!