You should strive to constrain your sketch elements. What I mean is using either one or more of the actual constraints, or dimensioning things, such that all elements are define into a single possible result.
For example, if neither your rectangle or your circle were define in any way, then when you gave one side of the rectangle a tangent constraint to the circle, Fusion has more than one option. It has to pick one of the following...
1. Move the rectangle towards the circle.
2. Move the circle towards the rectangle.
3. Enlarge the rectangle.
4. Enlarge the circle.
If your rectangle was already dimensioned, the #2 possibility is off the table, and Fusion has to pick one of the other three. If the circle was also already dimensioned, #4 is off the table also, and Fusion has to pick one of the other two. If the circle was also definitively located, then only the #1 choice is possible.
Make sense?