To confirm that it’s voltage you could extract inverter voltage and inverter power from say 10 am to 2 pm on a sunny day (when the inverter should be producing close to full power) and do an xy scatter plot with inverter voltage on the x axis and power on the y axis. If the data points follow more or less a straight line sloping down to the right then you have your answer.

The problem isn’t only on the Vector side, you are getting a reasonable amount of voltage drop/rise between your meter and inverter which could be reduced by fatter ac cable between your switchboard and inverter. You’ll need cables more than twice the cross sectional area to solve the problem this way (if it’s even possible).

If you end up talking with Vector another possible solution might be to ask if they will let you run an Australian profile on your inverter which generally allow higher inverter voltages before the inverter starts clipping.