In my model, the lines and nodes lying in the plane are not integrated in the surface in the surface properties.
When selecting the surface using the "Related Objects" function ("Edit" -> "Select"), only the border lines are selected but not the lines within the surface.
When selecting "Definition of Plane via 3 Nodes" option in the "Select in Plane" ("Edit" -> "Select") dialog box, the lines are integrated in the surface, but not when creating the FE mesh.
I have set the tolerances for regenerating the model as necessary.
Objects like lines and nodes are automatically integrated in a surface by default, as soon as these objects have a static function, for example supports, loads, etc.
Objects that are not statically relevant are not automatically integrated in the surface.
The background is that the results are not averaged at integrated lines, thus often leading to a jump in the color distribution of the results values (which makes sense for statically relevant lines). Many integrated lines, however, are so-called construction lines, and are 'just' imported when importing a DFX file.
In the "FE Mesh Settings" dialog box, you can specify the integration of all objects lying in the surface by selecting the "Integrate unutilized objects in the surface" check box.