I think you are asking if the gen input directly feeds a separate charger in the inverter...?Me, not knowing how inverter accepts generator power would think inverter manufactures
would only use generator to power battery. Something like a separate charge better.
That would make a desirable system.
Is this how inverter manufactures are designing? Any thoughts about this?
The "charger" portion in an inverter/charger is simply the inverter circuitry working in reverse to draw from the AC bus to charge the batteries. AC input, whether grid or gen, is just sync'd up to, then connected in via the transfer relay. (Really just a contactor.) The inverter is always monitoring the AC quality and if it deviates outside of acceptable range the transfer relay contacts are opened up again, and inverter resumes pushing power from batteries to AC bus.
Grid input vs gen input function the same, and pass power on through to loads whenever connected. The only difference is that the gen input is supposed to be less "picky" about the power quality, and is supposed to not allow "sell-back" out of the gen terminals.
That is the shortened, condensed explanation. The high frequency AIO inverters such as 18kPv and Sol-Ark have step up/step down dc-dc converters between battery voltage and high voltage DC bus. Then the actual DC-AC inverter runs between high voltage DC bus and AC bus. (Mppt inputs convert directly from PV to high voltage DC bus.) This extra step between AC and DC does make it harder for the unit to adjust to sudden power changes such as well pump, etc. shutting off and creating an excess of power on the high voltage DC bus, as multiple steps need to be taken to accommodate for the change.
Hopefully that answers your question, and helps to shed some light on internal operation.
One more note* I agree with you that if the UPS accepted the power from the Generac, it almost can't be "dirty" generator power, as UPSs tend to be pretty picky! Definitely would sound to me like something that may need to be worked on from the inverter side!
We did have one job where 3 Sol-Arks would push VARs to a 45kW genset (this was a 3 phase setup), causing it to actually throw an overload error! I believe that issue was then corrected with newer firmware that must have reacted better to the generator power, or stayed sync'd better.