style filtering?

Using the latest Calibre editor (3.44).

Usually (for the last two or three years I would say), when producing an ePub, I automatically join a second stylesheet, voluntarily oversized. Then, I edit the book by picking up and adding some additional styles or rules named on this second stylesheet. Last I remove the unused styles or rules, blend the two stylesheets and reorder the remaining one.

I was unable to work this way because the Calibre editor removed automatically all the unused styles or rules before I started modifying anything. It is very frustrating.

Is it an intended behaviour?