That must be the rosters you are using. I can guarantee that you will see fumbles still with it at 50. I'd love to have you over for a beer and talk sliders sometime and I will show you exactly what I am seeing.
I was skeptical at leaving the fumble slider high because on previous generations the fumble slider was for fumbles only. But now they don't have a RB ability slider and I swear its combined in the fumble slider. Where you have the tackle slider may play a factor in determining if there are fumbles or not because to some extent I think defensive run awareness and going for big hits and strips is tied to the tackle slider.