Navigation is constantly being rebuilt

We have one project and the navigation is rebuilding itself almost constantly with little or no input from the user. If we build lighting, it rebuilds. If we save the map, it rebuilds. If we package a build, it rebuilds. If we make even the slightest modification such as adding a pointlight, it rebuilds lighting. If we manually load a streaming level, navigation gets rebuilt

The map is 1 persistent level with about 12 streaming levels, each about 1KM x 1KM. The streaming levels are not automatically loaded, so when in the editor I have to load them manually to get around the map as they are loaded and unloaded in-game by players proximity.

When nav rebuilds, it’s not just rebuilding a small area, as it does when you change something(although when I move a chair for example, then it only rebuilds around that area). It rebuilds all 5000+ tiles each time this is triggered.

Any ideas as to what is going on here ? Why on earth does the navmesh for the entire map need to be rebuild once the lighting is baked? I have to assume something is a miss here