Can you share the code, a test while where this situation occurs and the values that you expect?
SOLIDWORKS doesn't highlight the bounding box when you only have sketches in the part design mode. Bummer!
I don't really trust the API's get box. It's not the tightnest fit. If and only if (this could extended to 2D sketch but then you have figure out the plane), you're working with 3DSketch, you can evaluate all the curves between their umin and their umax. Consider refinement of 100 points for example for each curve. You mostly need this for arcs and splines. After you collect all the points, the curves will fall in a box with defined by a lowest corner at (Xmin, Ymin, Zmin) and a highest corner (Xmax, Ymax, Zmax) where the max is the max of the Xs of all the evaluted points. Likewise for Y and Z. The only issue with this approach is how to determine the adequate refinement but overall, I found the results - I've used this for a customer- to be quite statisfatory.