Energy monitor - odd behaviour

If your electric motor is powering the vehicle, there are two things that would activate the ICE (internal combustion engine). If you accelerate quickly the ICE may turn on to assist powering the vehicle. Another reason the ICE turns on is if the windshield defroster is activated or the climate control is set to a temp that requires cabin heating.

 

In addition to what was just mentioned the ICE will run for a multitude of other reasons including hybrid battery temperature, a slight increase in power demand (which might occur if you aren’t using cruise control and you subconsciously apply more pressure to the accelerator; or will occur if you are using cruise control and it accelerates the vehicle or compensates for wind drag that you wouldn’t necessarily be aware of). Plus many more reasons I’m sure others will mention. It’s not actually unusual behavior.

 

In all Prius ( that I'm familiar with ) there is a spot in the engines rev range (rpm) where there is neither regen or assist and the car rolls without using power from either the engine or the motor. Gen 2 owners described it as heuristic mode. With a regular Prius like yours, it's rare that the engine wouldn't be providing power to the wheels at 100 km h.
Sure if you are going down a hill or on the downside of an overpass, that's when the EV portion of the monitor can display EV being used while driving normally at 100 km h.

I may not be understanding what you're describing about the monitor, but at first read it looked to me like it was backwards when talking about the engine and electric motor displaying on the monitor.