The “Navmesh needs to be rebuilt” text is flashing while the navigation mesh is being rebuilt at runtime. It is not shown when nothing changes. Why do I get this warning when everything works as it should? (“Show navigation” shows the proper, constantly updated nav mesh.)
Sounds like a bug. I’ve filed it as an issue to look at. In the mean time, don’t worry, this warning never shows up in shipped games.
Cheers,
–mieszko