Hybrid battery degradation

I have the car listed above hybrid which goes into electric vehicle mode at speeds up to 47 mph. I can no longer push the throttle down as far in EV mode before the gas engine engages. EV mode operates like it does when the outside temperature is really cold even when it is 80+ degrees outside. I have had the vehicle for nine years so I know how EV mode operates in various temperatures. I took it to the dealership to run diagnostics and they said the hybrid system is operating as normal. Is it possible that there is a hybrid battery temperature sensor giving a false reading saying that it is colder than it really is? The dealership said it could be battery degradation but I can still drive just as far in EV mode as I did when I first bought it; I just cannot push down the gas pedal as far without the gas engine engaging. It is extremely noticeable now when then temperature is below 60 degrees (F). The gas engine is on constantly except when I apply the brakes and then engages immediately once I let up on the brake pedal.