I’m having a similar issue, system started discharing at max power when the battery is full and I’m running in ESS mode. So not reason, at least in my view, to start discharing at max power and returning to grid. My expectation is that solar power is keeping the battery full/floating, the current load comes from solar and everything from solar generation which is left over is returned to grid.
My other post with more details: Running ESS and systems starts discharing 4kW, why?