When you create a door or window that extends to the base of the wall, the wall foundation breaks beneath the new element.
To change the default behavior, access the type properties for the wall foundation and clear the Do Not Break at Inserts option. See Wall Foundation Type Properties.
Note: The wall opening tool will break the wall foundation if the opening extends to the base of the wall. These openings are not affected by this parameter. See Cutting Rectangular Openings in Walls.