Both versions of Civil 3D 2018 and 2020 have the same problem. Polylines (LWpolylines, 2d and 3d polylines) do not display lineweights properly. This appears both on screen and when plotted.
Details/quirks:
2d polyline (heavy) appear correctly.
If the polyline has a fit or spline applied, it appears normally. If the spline is removed, the weight changes.
Civil 3D objects like parcel lines, feature lines, etc. that are derived from polylines appear normally.
If a polyline that does not appear correct is rotated or scaled, the entity jig displays the weight correctly but the weight changes back when the command completes.
If the polyline is exploded, the base entities appear correctly.
Does anyone have any help they could offer?
Thank you,
Stacy