Prius 2013 Hybrid Gear Shift Issue

I am having very strange issue with the Auto shifting of Gears. Gear will only change to the next when I leave the accelerator. If I keep pushing the accelerator without taking my feet out of it, the gear will not change. For example on straight highway, if I push the accelerator and don't take my feet from it, the gear will not change and I can feel from the engine sound that's its getting heavy but as soon as I will leave the accelerator and push it again, the gear will change and I can feel the car is getting lighter and the gear has changed. Anyone can please help what is this and how to recover from this issue and why the Gear is dependent on stepping accelerator. Appreciate it.

 

Thanks Maarten28 for the reply. Actually I have been driving it for last 2 years. How I found this issue as my fuel consumption has gone up. Before this issue I was getting 25Km/L and now it has drop down to 13KM/L. Also in hilly areas (uphill road) the car takes lot of power to go up however before it was very smooth.

 

Thanks 4est for the reply. That's an issue when I push the accelerator without taking my step out of accelerator, it remain stay at the current position (it does not revup) unless until I take my step away from the accelerator then it revup.