I could be wrong, but it looks like your dimensioning the distance between two points rather than the sketch line.
When you change the dimension, it just moved the lower point further away.
When I zoom in on the bottom of your part, I can see that the sketch line ends at the model, and your dimension leader terminates at a separate point not part of the sketch line.
Like perhaps you had a sketch line overtop of a reference line, and you dimensioned the reference line.