If you look at the grid voltages in the snap shots you posted, the L3 phase is up to 252.4vac when your only generating a few hundred Watts, so you're running close to the grid max. Maybe your inverters grid voltage limit has reset to 253vac after the power cut and you're hitting that? If you put a load on that phase, like an electric fire, it will pull the voltage down. Maybe add a kw or 2 of load to each phase when you want to test things