So if your inverter and battery are very close together just use multimeter set to DC volts, and measure from the battery positive terminal to the inverter's positive terminal, and then do the same on the negative side.
A difference in voltage would indicate bad connection or even a bad piece of wire (unlikely).
Voltage drops proportionally to current flow, so since you can only get 0.5A to flow through those wires, it would take a truly horrendous connection to result in the >2v difference. But it might also explain what's happening. Make sure the large metal bars you are bolting down to on the Growatt don't feel 'loose'.. if one broke loose from the board somehow that would be an easy 'stop here, repair/replace inverter' scenario.
If you don't find the voltage drop on 'your' wiring, you are probably right that there is a bad connection inside the inverter and if it is new you may just want to try and get it exchanged.
I totally agree with your diagnostic process so far. Checking that the BMS will allow 30a from a different source, good idea. Disabling communications, good idea. Changing battery settings to manual control, plus 'forcing' an EQ, also good idea.
@LydMekk , in this case trying to do a manual EQ is just trying to prevent the Growatt from immediately switching from the absorption charging voltage to the float voltage because it sees such a low current. At the end of the day with no more than 1a flowing we can call it whatever we want but we aren't 'equalizing' anything. ?