Bug or not? IfcTrimmedCurve problem

[GoTryCatch]

Hi everyone!
I faced the following problem! May be it is bug?
I draw trimmed curve IFC4 (arc of ellipse from 0 to 15 degree (Math.Pi/12))
Initial data: Ellipse (40x20) in original point (0,0,0).
I counted the desired point on the ellipse and know its coordinates.
Simple code:

var ellipse = new IfcEllipse(new IfcAxis2Placement3D(new IfcCartesianPoint(db, 0, 0, 0), new IfcDirection(db, 0, 0, 1), new IfcDirection(db, 1, 0, 0)), 40, 20);
var trimmedEllipse = new IfcTrimmedCurve(ellipse, new IfcTrimmingSelect(0), new IfcTrimmingSelect(Math.PI/12), true, IfcTrimmingPreference.PARAMETER);

But the result is not correct.. arc does not reach the desired point
Why is this happening?