The same thing happened to me on a trip on the 15fwy north towards Las Vegas on the highest hill climb through death valley, luckily it was nighttime during temperate weather, otherwise I may have been concerned. I took a photo of my battery indicating 0%. This was within one month after purchasing my 2019 PHEV as new, but it didn't happen on other trips up other hills, so that was an unusual circumstance. I still averaged 46 miles to the gallon on the trip to Vegas (from San Diego) over 320 miles, and 54 miles to the gallon on the trip back to San Diego (more downhill average driving) with average speeds of 65MPH using mostly Cruse Control (considered slow on that freeway.) Both directions of the trip was starting with a full battery charge. I did notice that the cruise control on this car is especially easy to adjust and control, in 1 MPH increments/decrements at a time. However, CC expects a longer distance between cars than you expect and sometimes "sees" a vehicle which is in a different lane as in your lane (during gradual turns) and will brake unexpectedly. I turned off lane keeping assist since it generally always makes bad decisions when exit lanes branch off.