It worked !!! The Runtime generation parameter (dynamic) solved the problem. The only slight downside: this change drastically lowered the FPS. I had to resize the NAV Mesh and optimize its location to increase the FPS. It’s not perfect (around 60fps) but it’s working !
If anyone else finds themselves in this situation, here’s what to do: delete all the Nav Mesh from the “sub-levels” and add one in the main map, and make sure the Runtime Generation is set on dynamic.
Huge thanks to @Leomerya12 , you solved a problem I had been struggling for weeks. I owe you a coffee ;