When a Landscape is edited with the Retopologize tool, Dynamic Navmesh is no longer generated. This was also tested with Static Navmesh and yielded the same results.
1. Set following options for the Navmesh
Runtime Generation: Dynamic
Data Gathering Mode: Lazy
Generate Navigation Only Around Navigation Invokers: True
2. Place a landscape, add navmesh, and edit using retopologize tool
3. Build navmesh
4. Setup a Navigation Invoker and place it on the Landscape
Results: Navmesh can't be generated on Landscapes that are edited by the retopologize tool.
Expected: Navmesh is generated
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-59153 in the post.