The Line Tool copies the details of the shape tools stroke and vice versa. This is great for adding walls and such but the line tool used to have a layer offset. This was removed as a mostly redundant feature, however now lines with the offset already cannot be changed. IF there is a layeroffset on the stroke and you then select the Line tool your lines inherit the offset. Whilst not a major issue as you can just realistically keep the line lower than objects you want it under, it's still annoying that we used to be able to change it but now it's stuck and old lines with offsets are permanently offset.

I doubt this is intended behaviour but it's likely that the offset slider for the line tool is necessary for parity of older maps being editable and for the sake of new lines that inherited an offset from a stroke.