Hey there @WolfTechRob! So off the bat the nav system (like the original character movement component) was coded with the ground being -Z all the time. This means it really only looks for things that should be walkable from that angle, so you’d have to roll your own navmeshing/ai to do what you’re attempting here. There are some examples in the community that have done something kind of like this previously.
An example is the SpiderNavigation plugin that used to be available (but is long deprecated) Spider Navigation in Code Plugins - UE Marketplace, and they have a github if you’d like to see how the old implementation was.
Another example is various forum posts over the years, Navmesh for spherical world
but none seem to give much of a hint into how they achieved it. Wish I had more to give!