I just posted another thread describing the exact same problem 
Thanks for the post! I just upgraded my project to 4.26.1 and my navigation wouldnât build during runtime, but it would if running in the editor. Every part of my project is dynamic and nothing can be pre-generated, Iâd checked over the official documentation and other forum posts but nothing seemed to push the engine to build nav data after packaging. After manually changing the file you suggested it seems clear the editor doesnât save a few options because it started working. I found out thereâs 3 different locations of the navigation settings, only 1 can actually be changed:
DefaultEngine.ini* > Unreal Settings Menu > Spawned Nav Data*
Each of those have the exact same navigation settings and each get overridden if the parent doesnât match (for some reason). In my case, the âdynamicâ, and the âmain nav dataâ setting. If you have an issue with dynamic navigation, navigation invokers, or just moving obstacles, try changing the appropriate navigation values for your project in the DefaultEngine.ini file, then re-package.
Similar issues. I cant get âDo Fully Async Nav Data Gatheringâ working any good. In editor all looks good (nav mesh generates around invokers), but once pressing play - only few navmesh tiles appear on level and those follow only very few of the registered invokers. If unchecking that async (even during play), then tiles generate instantly like they should (around invokers), but performance is much much worse.
5.0.3, and build navmesh only around invokers still donât work in packaged game. I did try all the suggestions in this thread.
Nevermind, it works, the problem in our case was the Steam build (or Steamâs patcher) not detecting changes in the defaultengine.ini file.