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Yes, a file would help
Please post the original Spline. The artifacts are due to coplanar intersecting polygons.
Make sure the spline is 2D. Put the Spline in a Loft and then make it Editable. Also post that result with visible polygons. Extrusion is a polygon modification not a Spline one, and extruding polygons with different normal directions can lead to unwanted results, even if it’s an Inner Extrusion.