As per the existing spec (comments), it is not clear what would be the value for JarFile#getVersion when JarFile API is used to process unversioned jar.

It is observed that when JarFile is used for processing unversioned jars, then getVersion always returns baseVersion.

It would good to mention this scenario in the getVersion spec.

something along the lines,
"assert !jarFile.isMultiRelease() && jarFile.getVersion().equals(JarFile.baseVersion())"