If you assign a Fillet constraint to an arc that has a line segment at each end, Parametric Design ensures that the angle subtended by the arc will never become greater than 180° by "flipping" the arc whenever this angle exceeds 180° in a variation. This adjustment is only made in the case of an arc with a line segment at each end.

If you intend for the angle subtended by the arc to be greater than 180° in the generated part, do not use a fillet constraint; instead, use two Tangent constraints to glue the ends of the arc to the appropriate elements.

Parametric Design fillet constraints give you better control than the Creo Elements/Direct Drafting Fillet Change command for changing fillet radii. By applying other constraints along with the fillet constraints, you can control exactly how elements tangent to a fillet adjust to accommodate the new radius.