Accuracy of Engine display in Energy Monitor Display

The engine is always at least spinning when you're at about 46 mph or above, though it may not be injecting fuel. When there are no arrows, it simply means that the engine is not producing any (significant) power that is being sent to either the wheels or the battery. It may still be spinning in fuel cut-off, or it might just be idling. From the stock gauges, there's no way to tell for sure whether the engine is off or not - that's one reason why some of us get a ScanGauge.

 

+1

This question has come up before with respect to the Gen II.
I suspect that the answer remains valid for the Gen III.

The ICE/Motor Generator/HV Battery/Wheel display is a simplification
of the real power flows. It has been called a "mimic," though I don't
know if that's a valid term in the formal sense of engineering/information
display. At any rate, it is not an indication of the ICE on/off status.

As a starter, the display cannot be wholly accurate as it only shows
only one MG, and at times one of the MGs can be generating and the
other motoring.

Secondly, in a "no arrows glide" there is a small power flow from the
HV battery to the MG. IIRC it's ~.75 HP which is not shown.