Navmesh won't rebuild at runtime in some areas only

I’m having an issue where some “Cells” of the navmesh won’t update at runtime. They only update if I force them to rebuild either in the editor or using the rebuildnavmesh console command.

231574-trees.png

Trees (these are foliage)

231575-treesfalldown.png

Trees removed at runtime foliage deleted and new BP versions in their place.

Here in another area of the map for some reason the navmesh does not update. You can see by the grid shape (in red) seems to be some of the cells never get update called.

Any help would be appreciated.

I think I discovered part of the problem.

I tried to repro this issue in TopdownExampleGame and after some experimentation found that the navmesh starts breaking if you add multiple nav mesh volumes. As you can see from this screenshot parts of the navmesh stop rebuilding. as you create new volumes.

Was able to confirm in a setup like this where you have multiple “islands” of nav mesh on the same plane it seems to stop updating as you would expect.

It doesn’t seem to be related to multiple volumes but instead to multiple “islands” of nav mesh in the same level. I was able to reproduce the same broken behavior simply by stretching a single nav mesh volume over all the geometry “islands” in the level.

Not sure why but it has something to do with the relative location of the other segments of nav mesh.

Moving my other islands of nav mesh as shown below also works around the broken behavior.