Not sure about the rest, but you are describing STANDARD AC coupling behavior that is necessary due to how grid tie inverters (GTI) work.
GTI do not regulate themselves. They expect an infinite grid in which to dump the maximum power the array will deliver - period.
When the 18kPV is providing the grid, it MUST absorb the maximum output of the GTI array. No exceptions, so the moment the batteries are full, there's nowhere for the power to go. The only choice it has is to frequency shift away from 60Hz to force the GTI to disconnect and switch to battery.
For comparison, Victron has the ability to interface with and throttle certain models of Fronius GTI. This is because the Victron ecosystem is in communication with the Fronius, and the Fronius has built-in throttling ability, which is not a standard feature. Additionally, due to the imprecise measure of this control, the grid is needed to buffer this function as the GTI is not sensitive enough to change output in response to loads. It still decouples from the GTI when grid is down. This feature exists to prevent significant grid backfeed when not desired. So for many thousands of dollars more, you still can't get your desired behavior.
So for you to get the functionality you want:
1) your GTI must be able to throttle itself.
2) the 18kPV must be able to communicate with and control the GTI.
3) the 18kPV will have to offer a function that no other AC coupling-capable inverter has.
Suggest:
1) Don't hold your breath.
2) Be thankful for the concept of AC coupling that gives you any GTI function whatsoever in a grid-down situation.