Don’t know if this is a bug in the nav mesh, or in the top-down dungeon content, but it seems to be the nav mesh.
Check out this picture (in version 4.5.1):
You can try this yourself by loading the demo map from the Top Down Dungeon content from the Marketplace, drag in a nav mesh volume, and set it to X 2000, Y 4000, and Z 500 height so it covers everything.
For some reason, the center of the corridor in front of the stairs doesn’t become navigable. There’s nothing really in the way there that I can see. I even tried moving the lights/sphere capture actors up, in case they somehow blocked navigation, but that didn’t help.
I also increased the step height (to 60) and actor walkable angle (to 49) to see if that was the problem, but no.