ICE turning on for apparently no reason...?

My 2021 Prime has turned on the ICE for apparently no good reason a couple of times, and not even for one of the many reasons listed in the user's manual.

Let me explain.
Today, it is 50F/10C outside temp, AC is set to 70F/21C, EV mode + ECO mode selected.
I drive the first 5 miles of a 7 miles trip with EV light on and all as usual and quiet and nice.
I reach a moderate, but long slope, reaching 50MPH going downhill. I start braking moderately (about 70% battery is left, used about 30% up to that point) and all of a sudden the ICE fires up and the EV symbol disappears (as it should).
I drive the next 2 miles at about 25MPH (as I reached the city center), and when I park the car at my destination, the ICE continues to run for a good 10-20 seconds more, until I decide to turn the car off.
I then turn the car back on 5 mins later, and the car starts regularly in EV mode and keeps it that way all the way back home, even going up hill.

Something similar, on the same trip on the same road, with the ICE firing up in the same place, i.e., while braking down-hill a couple of days ago, at about the same external temps.

I have done the same exact road tens of times now in EV mode, and the ICE never once turns on. Not even going up the same hill on the way back.

What is going on? why does the ICE turn on when the external temperature is nothing to talk home about (a very mild 50F)???
Is she having issues? or is it all "normal" and the ICE turns on because the battery is maybe too cold to accept regen charge at the rate you get when braking at 50MPH downhill?

 

This is a new behavior since getting the car in Mar. 2021 - and I repeat. The battery was 30% used already, so plenty of space to recharge it going downhill. The downhill slope is probably less than a mile long. So I don’t see how that could be an issue and required the ICE to turn on to compensate.

 

Today it happened again. Ambient temp 38F/3C, cabin pre-heated to 70F/21C while plugged in, EV mode from the get go. HVAC set to 70F/21C while driving in Auto mode, priority mode to front seats turned on.

Same road, same downhill after 3-4 miles of regular full EV mode, about 70% batter left. I hit the downhill, reach 50mph from 30mph (limit is 45mph), I start to brake slowly, adding to the braking the car does already regardless when you lift your foot off the gas pedal. And then, bam!, it turned the ICE on - and not to brake, as it was injecting fuel..!

The car continues to say it is in EV mode, but drives like as if it is in HV mode. I check the energy monitor and it confirms that (energy going from the ICE to the wheels).

I get to my destination, park the car. For 2-3 solid minutes has the ICE already being running. I wait, and wait, and it doesn't turn off.
I check the energy monitor, and no arrows are going anywhere. I think, maybe because it is in P (although, I thought, in P it should still charge the battery if the ICE is running; only in N, there is no charging happening). So I put it back in D (while pressing the brake pedal to avoid moving) and the ICE makes a different noise at it is not free-wheeling anymore, but is charging the battery (the energy monitor shows that).

I think "maybe it is the HVAC requiring the ICE?" So I turn the HVAC completely off. Nothing, ICE still running.
I press EV Auto, HV mode, EV mode - still running.

After probably a solid 2.5 mins, the ICE finally stops. It has been running at that point (i.e., since the downhill), at least a solid 5 mins, probably closer to 7 mins. That has nothing to do with warming up the ICE. In the Gen4, even with the HVAC depending on the ICE heat, the ICE would heat up from a cold start in fat less time than that.

I go and do my errand (about 5 mins), get back to the car, turn it on, EV mode, I turn on the HVAC back to Auto/70F/21C (the ICE doesn't fire up). And then.... I drive like nothing has ever happened, all the way back home in pure EV.
I even take a different route after the uphill (about half way of the whole trip), that takes me to a faster (55mph speed limit), longer downhill. I brake less, I brake more, along it: nothing, the ICE never turned on once.

This leads me to believe that the car is doing fine (otherwise it would have started the ICE again; ambient temp was the same) and the time to "heat up" from a full stop (while parked), was about the same as when I first gotten to the downhill earlier on.

So I am wondering: what technical requirement, does the Prime have, to have the need to turn on the ICE and run it for almost 10 mins, at a mild 38F/3C (because 38F is honestly not *that* cold for a modern car)? What is the trigger of going down a mild downhill, after running in EV mode without issues for the 3-4 miles prior to that, from a total cold start? Why did it not start the ICE from the get go, if it was "that cold"? Or why did it not start the ICE at the first of many smaller/shorter downhills I get in those 3-4 miles prior to hit "the" downhill, where I do need to brake a bit?

This behavior seems to have absolutely no logic or a clear technical requirement. I am honestly puzzled...

PS: the one last test I can do is to get the car through a similar route, but at the mid-way point, before going the infamous downhill, turn the opposite direction at the traffic light, and drive down the much steeper/faster downhill and see if the ICE fires up. It should, but I would still like to know why...

 

Just checked the manual and, essentially, EV mode is not a guarantee that the ICE will not turn on, and the reasons why the ICE will turn on are sort of "randomly":

 

Well, if I was worried about the car being "broken" now I know that is just some of that Toyota engineer's weirdness...
I find interesting in the manual the statement that the ICE will turn on "When the temperature of the hybrid system is low."
I would then expect the ICE to turn on as soon as I turn on my car from a complete cold start (i.e. after a full night), instead it does that after 3-4mi of EV driving, at the first downhill when I brake "too much"...

 

And today she did it again...!
48F ambient temp. 70F AC setting. EV mode. After 5.5mi driving on the same route as ever, mostly uphill, I hit the downhill, I start picking up speed, reach 50mph, I start gently braking to slow down to 40mph, and bam! the ICE starts. At the first traffic light I reached, I turned off the car, restarted it and the ICE never again turned on and used the battery down to 39%.
What is going on? What is the very specific condition that turns on the ICE at the same exact point. It wasn't even that cold.
I don't get it. And it cannot be a fault or a technical problem, because it happens always at the same place, on the same route, but not on other routes. (at least that I could notice).