Sorry for the late reply, Today I tested to manually feed back into the grid. I have set all 3 Multi RS to feed back into the grid with 6000W, plus I also have 8000 watts of PV power at the moment (SolarEdge 3phase PV inverter on AC-in), however it seems that just like in your case, the power is limited to 5000watt per phase, so about 20,5A per phase.
This is when I disable DESS, but I noticed that when DESS is active, this limit is not enforced as I have seen over 16.000watt in total feed back to grid.
So I am not sure were the problem lies. But my guess is with the Multi RS firmware. I am running firmware 1.20
