On-grid domestic battery storage

Qyburn

The 2012 system remains separate and as installed, but the hybrid inverter 'sees' the export, when that system exceeds household consumption, and uses it to charge the batts, alongside any PV generation from that side too

Thinking this through I can't see that behaviour specifically documented, nor a setting to control it. I don't have a separate system to check, so can only make a guess. My guess is that it's inherent in the default mode "Maximise Self Consumption". In that mode my system tries to maintain import/export at zero by varying inverter output, with surplus solar going to the battery. The behaviour you see suggests it goes further, dropping inverter output below zero and charging from AC if needed.

It would be interesting to see if that AC Charge Policy affects this bebehaviour.