Hi Mat, so it's doing that because the 2 edges are at slightly different z levels. So when the Planar command looks at them, it finds 2 separate planar curves to fill in, instead of one common plane with inner and outer boundaries.

If you select these 2 edges and then zoom in to the front view a bit you can see there is some separation there:







So the problem is that these 2 surfaces in your source solid are not quite on the same level:



Do you have any earlier stages saved so you can show me how that area was created?

- Michael