If the level has hidden sublevels, the regenerated navmesh result appears to have a hidden sublevel mesh placed at the origin.
Apparently, the reason is that bNavigationRelevant is no longer considered when exporting the mesh since 4.24.
The following workaround worked
Incorrect navmesh is displayed in center of the level (0.0, 0.0, 0.0 ).
please see screenshots below.
1. opend level (sublevel is hidden)
2.moved a mesh and an incorrect navmesh is regenerated
3. once the sublevel will be visible
4. If hiding the sublevels again, the navmesh stays normal .
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-102671 in the post.