Jonathan Mack

the border line is due to needing neighboring pixels to compute the slope. Along the border there are no neighbors, so the border condition is used to define what the neighbor would be.

Constant uses a single value for any sample outside of the boundaries, regardless of where. This would cause the slope to be equal to the value of the border pixel--probably not what you want.
Wrap will use the pixel on the opposite side as the neighbor. The slope depends on how different the opposite side is-for tiled patterns this is what you want for anything else it will be unpredictable.
Clamp/streak will extend the border value out to infinity. causing the slope to be zero on the edge, this is also probably not what you want.
Mirror would bounce back from the edge and use the neighbor in the opposite direction to calculate slope. The slope would then be streaked for a pixel along the border. this is probably the least egregious option. I'm not sure if mirror exists in COPs as border condition though.

It’s more likely a menu bug, and an old one at that. When a menu contains dynamic options, and only one option exists, and the default option is not in the list of options, the menu will show the only option as selected but really the invalid default is selected. This is usually worked around by opening the menu and clicking on the one option. Does that fix it? If not then it might be a bug in the node not respecting the chosen option. What does it say if you right click the parm and open the expression editor? Does it say “optix” or “embree”?

Can you just get the magnitude of the directional slope field?