Hi jki,

re:
> In one step, adding a detail the solid is "broken" to a joined surface... despite the operation does NOT
> affect the areas which are now marked as naked edges.

Non-manifold or self intersecting geometry can affect operations on other areas of the model too. Particularly mechanisms that involve traversing connected faces.


> So it means there is something wrong with faces... Is there a way how to identify it and prevent such situation?

Is this on the same model that you posted above? Because like I wrote above, the thing that is wrong there is having places where 4 surfaces are touching at a single edge making a non-manifold structure.


> Is there an the explanation why in some cases the "boolean Union" end up as Boolean Difference?

One of the things that non-manifold/self-intersections can mess up is the algorithm that identifies which side of a closed skin is the outside.

If it ends up classifying the wrong side as the outside it can invert which pieces are kept and discarded in booleans.

- Michael