23' Prime - Gas engine starting in EV mode

I don’t know an exact number but it is not that low. I don’t have data readily available to support this but am pretty certain that when I am driving on a highway using cruise control lets say set to 75 mph and then approach my exit and quickly reduce the cruise control to say 30 mph I see a regenerative power close to -75 kW.

My bottom line for now that is in the current Gen 5 Prius Prime the data suggest that the engine starts after regenerative braking most likely due to drops in engine oil temperature. The reasoning is that the car is traveling at high speed with air flowing through the radiator and causing a drop in engine oil temperature. The gas engine starts to heat up the oil.

This is different from previous non plug-in Prii with smaller batteries where shifting to B when the battery was full resulted in a negative torque value from the motor thus slowing the car via engine braking.

 

What green marker?

 

Maybe we could also check what happens after the ICE has already run earlier in the trip.
I wonder if, in that case, the engine would stay off during regen since the oil should already be warm enough.
That could help confirm your hypothesis even more.