Do you mean you are changing the slider maximum value at runtime? That's unusual - why do you need to?
Probably the component developer was only expecting the Thumb position to be changed at runtime. The work-around in your case is to move the Thumb to suit the ratio of the new slider maximum.
.... it's your App, your GUI - but that is an unusual use of the slider, given that it does not display values so you still have to use a label for that. The App Inventor slider is for the input of values rather than the output. Still, you do understand that if you change the max value you have to also change the thumb position to match the new value range.
So i refresh the value, of course, and thumber will goes to 19 little to little, value by value, but first it position with no coherence. If thumber value is one, how can it be at the middle of the slider?
It is - certainly not the intended use - which is why I asked if it should be there. It's always a good thing to keep your App and App Code as simple as possible.