Engine turns on after motion stops

I've been searching the internet and striking out on this particular symptom - the closest I get is a few discussions on this forum, so I figure this is the place to ask.

2015 Prius, 125k miles

As the title says, my ICE has developed a habit of starting AFTER vehicle motion has stopped. The particular conditions that make this weird:
- It happens after I've been driving long enough that the ICE itself is warm, and is not running to warm up
- It happens when the weather is warm enough that the ICE is not running to provide heat to itself/the cabin (and it happens when the cabin HVAC system is off, just to check that off the list) [I live in Alaska - cold is the only car-hampering weather we have - it's never car-stiflingly hot here]
- It happens when the battery is charged in its normal range ~4 to 6 bars (I know the ICE will sometimes run when the traction battery is too low or too high, and that is NOT the case here)

The scenario: I'm driving along on a city street, often slow enough that I'm running in EV mode, or at least slowing down gently enough that the ICE turns off as the vehicle speed drops below ~45 mph, then I come to a complete stop with the ICE off, and a couple of seconds AFTER motion has stopped the ICE turns on.

Sometimes I can hit the "EV Mode" button and the ICE will turn back off - but sometimes the computer won't allow EV mode, and if I don't catch it immediately as the ICE starts, this does not work. Sometimes I can put the car in park and the ICE will turn back off - but again, this seems to work best if I do it immediately as the ICE is spinning up. If I turn the car off and back on again, the ICE will stay off until I move.

This started approximately a week before my last oil change. I relayed the symptoms to the dealership, and they speculated that my 12V battery was bad. They tested the 12V battery; it was fine, and they didn't trouble shoot further.

There have been two other slightly odd intermittent items. (1) Under normal conditions, unless I'm on a prolonged downhill, the car does not charge the traction battery beyond 6 bars (approximately 3/4 charge). Several times recently, under ordinary driving conditions (not going down a hill), my traction battery gets up to 7 bars (one from the top). And (2) this has happened exactly once, but I had an episode where my cruise control quit working and the ICE would not turn off at all. The next time I turned the car on, these symptoms vanished.

My car is otherwise operating normally.

I have wondered if this is a sign the traction battery is failing - but other people's symptoms for traction battery failure seem to be significantly reduced mileage, bad ICE performance, wild swings in traction battery charge levels, and dashboard warning lights. None of that is happening.

I've ordered an odb2 code scanner, which will be my next bit of trouble shooting.

Thoughts? If your ICE turned on after you stopped, what would you do?