Sine Wave Inverter Anomaly

I don't consider the OP's inverter behavior to be an anomaly, but rather a characteristic of a higher-end inverter. The better inverters have a floating output that can be used either that way, or bonded to ground. Possibly all TSW inverters by their nature are capable of having a floating output. The higher end MSW inverters have a floating output, while as noted in the previous post many low end MSW inverters have both outputs "hot" with respect to ground. It simply takes more circuitry to isolate the AC output from the DC input.