The power does have some place to go, the grid. Looking
at the graphical displays of power flow. The power from the DC coupled array goes to both the load and the grid
the huge key difference that you are missing here:
The integrated MPPT's are able to communicate with the rest of the sol-ark, and it can make intelligent decisions about how to handle the power the PV is generating. INCLUDING selling that excess to the grid.
This communication simply doesn't exist with the additional MPPT. It's quite literally a dumb device, as has been explained multiple times. It's *only* job in life, is to charge the batteries up to the settings you specify in it's config. Once the battery is at that level, the CC has done it's job and will back off until it decides it has more work to do. Based on your comments, I'm 99.999% sure this is whats going on.
The *reason* we've mentioned setting the sol-ark to sell battery power to the grid, is so that the battery voltage drops a little bit, so the dumb CC can see a need for power, and "wake up".
If you are concerned about it charging the batteries from utility.. disable that setting. Let the extra PV and CC charge it, as it's intended.
I specifically mentioned TOU, to allow the inverter to sell from battery to the grid during sunshine hours only, so it can't deplete your batteries overnight. This allows your batteries to drain a little bit while the sun is shining, and allows your additional CC to do it's thing. but when the sun goes down, stop selling from battery to grid.
But yeah man, step away from this for a bit and clear your mind. then, re-read and see if it makes sense.