I have noticed the following - over the last three months the Multiplus measured voltage has been drifting up. At the beginning of August the Multiplus voltage was within 0.05V of the BMS voltage. Three months later the Multiplus voltage is now 0.8V more than the BMS voltage. I have confirmed with a quality multimeter that the BMS voltage is indeed measure correctly.
This has some peculiar side effects in that when the battery is 100 % charged the system starts oscillating between 99 and 100 % - I presume because the Multiplus is letting the battery voltage drop too far because it thinks it is really higher.
Has anyone else had this issue?
The closest I could find is in this thread: https://community.victronenergy.com/questions/138444/multiplus-voltage-differences.html - user @anieuwstadt had a similar issue that was apparently an "inverter hardware" issue.