When drawing a line and a plane that intersect each other, the line might show up as two lines. especially when there is a high contrast between the colors.

This is probably due to the method used to draw lines (i.e., two triangles to make a quad) and planes being the same.

Somehow, while sorting the triangles by the ZBuffer, we suspect that the sorting algorithm doesn't take into consideration the order in which lines or planes were created, leading to a reordering of the triangle that makes part of the plane be drawn between the line quad.

or,

That is because the line quad is always being drawn perpendicular to the camera, the plane is not at the same angle has parts that cross the line plane.

The part with the difference in Z-axis value from the camera would show in the middle of the line quad.