SCR_ExportGeoJSONTool.c, line 462:

GeoShapeType shapeType = GeoShapeType.UNKNOWN;
if (classname == SCR_GeoExporter.POLYLINE_SHAPE_ENTITY)
{
	if (isClosed)
		shapeType = GeoShapeType.POLYGON;
	else
		shapeType = GeoShapeType.POLYLINE;
}

This code sets the shape type from UNKNOWN to POLYLINE / POLYGON, if the shape is a polyline. If the shape however is a spline, it will remain UNKNOWN and consequently, an the entry is not exported.
Instead, the surrounding if statement should be changed to this, or removed completely (since a previous if already checks this):

if (classname == SCR_GeoExporter.POLYLINE_SHAPE_ENTITY || classname == SCR_GeoExporter.SPLINE_SHAPE_ENTITY)

This simple change would allow modders to export their splines as polylines, which is usually close enough and always better than nothing.

Optionally, a warning about this could be added.

if (classname == SCR_GeoExporter.SPLINE_SHAPE_ENTITY) Print("Exporting spline shape as polyline!", LogLevel.WARNING);