[UAHaerospace]

I had a fault starting to happen more and more often several years ago, turned out that one of the internal parts of the Inverter was going...so I replaced it with another same make model with 3 year warranty - no issues since.

now, that's not necessarily your problem, and I dont' really think it's actually any 'DC' fault since that DC is simply referring to the Voltage reading of the batteries, while being charged by shore power.

as to 'why' things went off when you turned the Inverter OFF, you are correct that they should not if you are on shore power, as leaving the inverter ON is simply allowing for the inverter to 'react' if shore power is lost, and start providing power to those outlets. If you lose power to your pass-thru powered outlets, then yes, it is not acting correctly.

If you simply reset everything and it's back to working correctly, especially if the pass-thru powered outlets are working from shore power even with the inverter OFF, then you solved the issue, at least temporarily, like 'rebooting' a computer that just starts doing something strange, for no reason.