Prius auto shifting into neutral

Hey Guys,

So I have a 2008 prius with 110k miles on it. Recently, my car started randomly going into neutral after short commutes. It would most often happen after coming home from work (12 mile commute), and as I go to reverse in the driveway, it would go into neutral. I would then have to hold the shifter in reverse. I notice this happening on hot days- never happened in the early AM when the temperature is still fairly cool.

I checked the inverter pump, and it had indeed failed, so I replaced it with a new pump and did a drain and fill on the inverter coolant with Super Long Life coolant from Toyota. After replacing the pump the issue went away until we got another heat wave. It's now having the same issue where it throws the prius into neutral, most often at the end of my commute when reversing into my driveway. I checked the inverter pump and it is humming normally and there is turbulence In the reservoir. I bled the coolant thoroughly after replacing the pump and old fluid.

I only ever had this happen once WHILE driving, and it was because i used the regenerative breaking. I did this assuming it would add load to the inverter, and I wanted to see if the inverter overheating is the culprit to my problem. After a few seconds in B mode at around 50mph, the prius threw itself into neutral. So I assume my problem is the inverter is still overheating?

The odd thing is, I never once got a check engine light or red triangle of death, even before replacing the failed inverter pump. Not a single warning light. Not sure how to start diagnosing this issue anymore, as from what I can tell, the new inverter pump is working fine.