Any label component that is drawn before the mask may be covered if it is within the mask boundary.
To set the background mask setting for surface contour labels in AutoCAD Properties
In the drawing, right-click a surface contour label and select Properties.
Do one of the following:
In the AutoCAD Properties palette, under Labels change the Masking property to From Label Style. This sets the contour label to the label background mask setting from the assigned label style.
Under Labels, change the Masking property to Contour Line Only.