We are experiencing this same issue on multiple sites. Both with AC connected PV and sites with charge controllers.
Quattro 5kw off grid system. 50hz 230v
When the backup generator is operating, the output from the Quattro varies wildly and also it appears so does the load placed on the generator - to charge the batteries (from the VRM portal data)
This is backed up up by the display on the generator (Hyundai DHY6000) which shows frequency.
Frequency output from the quattro is so unstable it cases the Fronius PV inverter to cut out and also the washine machine on site to go into fault. Also voltage is all over the place, and all this causes lighting flicker too.
UPS function setting is off
I have tried lowering the shore power setting.
I have tried lowering the output voltage slightly on the Quattro. (output on the genny when unloaded is 230v)
Dynamic current limiter on or off makes no difference.
Accept wide frequency range is on.
I am assuming that the quattros inverter attempts to track the AC input frequency and doesnt do a great job of it.
If powerassist was disabled - would this prevent the inverter output from trying to track whats happening with the AC input ?