AC300, PV in = 0W & B300 @ 100% battery question

If the AC300 has a constant AC Load (e.g. 300W), and the B300 batteries charge up to 100%, the PV input drops from 1000W+ to 0W. At this point that 300W AC load “SEEMS” to be running off the battery, until the battery level reaches 99%, then the PV input goes back up for a short time until the battery level displays 100% then the PV wattage drops to 0 again.
My question: I thought that as long as the PV in is greater than total consumption, then the MPPT controller “passes” the wattage being generated by the panels to the AC300 inverter without “draining” the batteries, therefore the PV in should be at least equal to AC Load + inverter consumption (NOT 0W). Is there something that I am missing or is this misunderstanding maybe something that is a limitation of the app? Or are the batteries actually being used once they reach full capacity, while there is plenty of Panel Power being generated.
Thank you for any insight,