I always top up the tank prior to storage. I average pretty close to 50 mpg. However, on the first fillup of the new riding season, and only the first fill, I'm noticing a significant reduction in my fuel milege. As low as 26 mpg to 38 mpg.
All the fills after that have the mileage hovering in the 50 mpg. range, give or take.

I'm at a loss........................Dave