I tried messing around with the navmesh generation settings but nothing fixed this. The models use per-poly colision. It’s definately not a problem with the models, as it works on each of the few models but not on each of the many actors.
I tried that. No luck. What else I tried is putting down the sections of the dungeon by hand. That works as intended.
So the problem is somewhere in my dungeon generation process. There I noticed the navmesh doesn’'t appear to update when “set static mesh” is executed. Even when I rebuild the navigation afterwards via the right console command.
I solved it by automatically moving all my meshes a tiny bit and then back to the original location once the dungeon generation process is done - that updates the navmeshes which didn’t update when static mesh components were changed or added.