When a bone is constrained (a slider is a constraint), if the Mix of the constraint is 100, then the constraint will be applied fully. This is the same behavior for any constraint: https://esotericsoftware.com/spine-constraints#Mix
To be able to control the bones you may set the Slider Mix to anything less than 100, or check Additive
in the Slider properties in Setup mode, so that the bone transforms are added to the existing pose instead fo replacing it.
So to answer, yes this is normal and expected.