When used directly in conditions, System properties aren't set, either in 4.3.0 or 5.1.2, on Windows 10. Either when using 4.x or 5.x style (SYSTEM_ or SYSTEM). Seems like they are being initialized after the xml is processed.
Works fine on Windows 8 and non-Windows platforms.
Workaround: Use as the value in a variable in the dynamicvariables section. This fixes it both on 4.3.0 and 5.1.2.