The IEEE C37.118 standard is currently the single most ubiquitous synchrophasor protocol. The standard has been implemented by many different PMUs and PDCs, including the openPDC, and every one of these devices have been able to successfully communicate phasor values. For a long time now, the PMU Connection Tester has often been used as the gold standard to test whether a device has implemented the protocols properly.
So please trust me when I say that if the PMU Connection Tester says your voltage is 0.29 V, that means you have encoded a value of 0.29 V into the data frame. I personally do not think you will be able to solve this problem without gaining a better understanding of the IEEE C37.118 standard as well as the MATLAB code that you have used to encode those data frames.