Great question — answering as the UphillTech team:
I'd bet on a double neutral-ground bond rather than a bad GFCI. A GFCI trips when the current coming back on the neutral is about 5 mA less than what went out on the hot, so anything that lets return current sneak back through the ground wire or a second neutral path will trip it instantly. This explains why it behaves on inverter power and not on utility. Most MPP-style hybrid inverters only create their own neutral-ground bond while running off the battery. In bypass they just pass utility neutral and ground straight through, and the bond is made at your main panel. Your tester showing "open ground" on inverter power fits that: no bond at the outlet, and the GFCI is happy because nothing...
Full answer by Fusion Sun Solar: https://diy.stackexchange.com/a/333560 (CC BY-SA 4.0).
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