some polygons disappear when applying sin function in Curve.Offset, Curve.Offset polygons are in blue and the original polygons are black.

@abdelgawad See if using a code block instead of the formula node helps.

The formula node seems to be throwing up extremely large values and the generated geometry is probably very large (and hence seem to disappear)

a*Math.Sin(b*x);

@abdelgawad this has been discussed before on this post Why Math.sin values are different from sin values in formula?