EV Mode Cutting In/Out?

So, I took my brand-new Prime with me to work for the first time today. Due to a muddy front yard, I wasn't able to charge it last night, but I still managed to go 12 miles on the 6.5 miles I had left. Bear in mind that I live in a hilly area.

I charged at work, and left with a full battery. About a mile down the road from where I work, there is a long downward hill. I went down, using the regenerative brakes on the way to recapture what little energy I used in the first mile.

About halfway down the little ev green car disappeared from the display. I figured it was due to a full battery or something, as the energy display wasn't showing the engine running. However, I got to the bottom and went to keep driving when the engine started providing power to the wheels, not the motor.

It took another few miles before the EV kicked back on, and only after I switched it from normal mode to ev (even though I never switched it out to begin with). I drove for about another two miles, going up a decent sized hill in EV. I went down another small hill, regenerating on the way down, and it cut out the EV mode again, and used the gas engine to bring me up another hill. By the time I had gotten to the bottom of that hill, I could run in EV and did so for the rest of the trip.

It was a nice day, so I didn't use any air conditioning or heating, no heated seats, and the defroster wasn't running. I had 20+ miles of charge left on the battery, so I am confused why it kept cutting out on its own? I've gone up and down steeper hills while keeping it in electric, so why did it cut out on these hills?

 

I think I will try that. I have a 2017 Prime Plus (Haven't updated my profile). "Normal Mode" is what it's in when not in either Eco or Power (the system literally calls it "normal").

I'm going in for a short shift today (only 3 hours), so I will see if the difference in charge will affect it. I'm also going to try something else, since last night on a different route it did the same thing again without a full charge, albeit it was nearly full.