That's odd. I've seen quirks like this with PowerCLI in the past. Perhaps try a perl version to validate? The vSphere client basically uses the API, so it would be odd for it to work via the vSphere Client and then not work via the API in other languages, but PowerCLI does a lot of data manipulation with the cmdlets that may be bugged.
You could also validate the versions through the MOB interface vs writing another script. If they are the same via Perl/Java/dotNET and the MOB, then it should be reported as a bug.