I have also seen this kind of MPG fall off on my 325mi ( one way) trips to the Gulf even tho I am going from an elevation of 950 ft to 65 ft ( on top of a dune). My take on it is that the HV battery gets warmer just sitting in the parking lot of the restaurant and moves on to the 'warm battery algo' which means not as much capacity when running in hybrid mode, more ICE running during the 2nd half of the trip. I think that I should do a grid-rebalance on my HV Battery to improve the capacity as it has 132,000 miles on the clock.
I just had a wild idea. Is it possible that you were watching one of the "average" mileage displays that gets reset every time you stop the car and re-start it ? Or the one that automatically resets whenever you put gas in the tank ?? As opposed to the real average that does not get reset until you do it manually ??