Faulty knob
I had already bought another Bluestork mouse and had the same problem.
The scroll wheel seems to be defective, after about 4 months the problem appeared, and now it's been 2 months and it's getting worse and worse. When I go down with the wheel, the screen goes down, up and stays there very anarchically. I can spend literally 5 whole seconds just trying to get to the next page of my document.
For the price, I guess this mouse does its job and you can't ask for more. Great for troubleshooting, but don't consider this model for long term use.