The reason is that there is no capability to specify which battery the Orion is charging. It just assumes (with no ability to override), that it’s charging the same battery that the Cerbo is monitoring.
For example, on my boat I have Lynx BMS with Cerbo monitoring my 600Ah house bank. And the Orion goes FROM house bank TO engine battery.
So if I connect Orion to Cerbo/DVCC, it automatically assumes the opposite — that I am charging FROM engine TO house. And this is where the reported “16.2v bug” comes into play, since it gets confused about voltages thinking the house is engine and engine is house. So it tries to raise voltage up to a maximum of 2v over the acceptance charge (14.2), and then stops charging.
See this topic for more info: https://community.victronenergy.com/questions/308611/orion-xs-stops-charging-and-sets-output-voltage-to.html