When you generate a fillet in a sketch in Fusion 360, it generates a tangent arc to the two specified lines at the specified radius and truncates the lines.
This is almost guaranteed to screw up if ever you move those lines, as might happen with a parameter adjustment.
This also means that anything that has a relation dependent on the junction of the two lines will have that relationship broken.
The expected behavior is that a fillet is it's own entity that manifests visibly as the arc and truncations, but actually maintains the integrity of the original two lines including their length and potential junction point. When those lines are moved or recalculated for any reason, the fillet should also be recalculated from the two lines in their new positions just as if the user was creating it again from scratch.
Essentially, Fusion should know what a fillet in a sketch is at all times, not just as a shortcut for making an arc and truncating some lines.