Hi Petr, here is a bit more description on how the style assignment for new faces is supposed to work. This is for cases like with Fillet where there is an already existing object that is being modified with new pieces getting created inside of it.

Say we have a shape like this which is 1 solid but is colored with 2 different styles on different faces:



Now say you select these edges and Fillet:



If the new fillet pieces are surrounded on all sides by faces which all have the same style, then the fillets will take on that style as well, like this:




So that makes things like fillets try to adapt themselves to whatever styles are predominant in a particular localized area of your model.

If the new faces are surrounded by multiple different styles instead of one consistent one, then in that case it will use the parent object's style for the new faces. That's the one that will get set by the first selected object when you do a boolean, or also you can set it manually by holding down shift when clicking on a style swatch with the solid selected.

- Michael