Mileage drop on 2006 after 12 volt battery install

I own a full loaded 2006 Prius purchased used one owner in 2013 at 80K , current mileage 165k. I had a new 12 volt battery installed at Toyota dealer April 2021 this was the second 12 volt replacement first one was about 6 years ago . I had been getting a steady average of 44mpg when , since the battery change it dropped to 36-37mpg - recent road trip in 95 degree heat brought it up to 37.6 the highest I have had since April . I assume my hybrid battery is showing signs of failure, the car is 15 years old . Is there any correlation with the 12 volt job ? Or just coincidence. I don’t want to start shopping for a new car, just got new brakes tires and battery it’s good to go and used Prius prices are inflated right now .