Looking at the circuit diagram, it seems current is being supplied from the output through the parasitic diode of the high-side MOSFET, causing the converter to operate.
Since the output is 3.0V, it enters 100% mode. To raise the output voltage, the high-side MOSFET is continuously driven, consuming about 400μA of drive current. Once the output reaches 3.3V, both the high-side and low-side MOSFETs start switching normally, reducing the consumption current.
This is my guess, but I think it’s pretty much the same as yours.
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