Placed a large platform at y75 using the fill command.  When I realized it was placed in the wrong area, I filled it with air, adjusted the coords and replaced the platform using the fill command in the correct area.  The area where I removed the platform still shows the shadow under the former platform as if the blocks were still there. It has the shadow look, but mobs can still burn up.  This is a brand new world created to test things, not a previously created world.