Whenever launching the UE5 Editor, it rebuilds the navmesh on the first map it opens. Reopening the same map again will not cause the navmesh to be rebuilt. It seems that the first time the map is opened, the addition of objects occurs late and marks the DirtyArea as NavOctree is updated.
1. Open the attached project in editor and launch with NavLevel_00.umap.
2. Open other map. (e.g. NavLevel_01.umap)
3. Reopen NavLevel_00.umap map.
In Step1, the navmesh is rebuilt immediately after startup. However, in Step3, the navmesh is not built when the same map is opened.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-170757 in the post.