Quattro (120v Single Phase) will sometimes fail to go to inverter when disconnecting shore power

DIY
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When disconnecting shore power in my trailer, the Quattro will sometimes just shut off when AC power is removed from the Shore Power input. It should switch to inverter because the UPS mode is enabled. The battery is a high quality 24V LiFePO4 rated at 7KWhr. When the failure occurs, it is when the battery is fully charged – usually 27.2 to 27.4 V. The last time the problem occurred, there was only one appliance drawing power (approx 400 W). The temperature is well controlled and is typically 80 degrees F (monitored with a separate battery temperature sensor). This has only happened three times during the last seven disonnects. I disconnect power using an internally mounted high-current switch so the problem is not related to any problem with a shore power circuit breaker. The Quattro will only start up again when I connect AC power to the generator input. After that, it has always switched back to inverter mode when I remove AC power from the generator input.