I had this exact same issue. In trying to fix the issue, I rejiggered the navmesh, to no noticable effect. Then I realized: in testing the cell, I had saved my game at a spot where my character was already inside the cell in question. Since, in that save point, the draugr was already standing on top of the coffin, he would still be standing on the coffin in any subsequent loadgames. I reverted back to an earlier save game wherein the cell would never have been previously loaded and then coc'd into the test cell and bam! the draugr is lying down in his coffin. I hope this helps someone.