Missing outer wall in a lithophane

[mxm_g 0]

I am trying to get to the bottom of why there are lines across my lithophanes sometimes.  I am attaching a photo of layer view in cura where you can see the outer wall missing in certain layers, including the layer at the very top and about 15 layers down.  I am printing wall thickness 12.  Changing infill setting does not affect this.  https://imgur.com/a/8VSyC0n