Hello gfinch,
If you used "enabled interpolation" option during the DEM generation, it would interpolate the surface in the areas that are seen from at least one camera. You can disable interpolation, but it could result in holes in the elevation model.
In case you require to have the exported orthomosaic with the straight edges, I suggest to draw a polygonal shape that includes the area of interest in the Ortho view mode and set the Boundary type for that shape.
To have the DEM and orthomosaic cropped to shape the following workaround can be used: draw boundary shape, export DEM, import it back and replace the existing one, then generate orthomosaic using the cropped DEM as a surface.