There are two setting for the angle of polarization Phi: absolute and relative. If the measured angle of linear polarization seems to be off, e.g. vertically polarized light is measured with an arbitrary angle to the vertical alignment of the device, then the setting might be set to "relative".
Please check, if the angle of polarization Phi is displayed with the suffix “rel.”. Then change the setting from a relative angle to default setting via “Edit” -> “Set Azimuth (default)”.