Well Roger, we both speak French but in this section I'll answer in English of course
. By looking at the screenshot I can see that Tidy is activated (little broom icon) and that's why Sigil creates a specific class for bold and italic (sgc-x, x standing for a digit). So, not really a bug, it's just cleaning.
That said, that type of coding seems strange to me but that doesn't mean you shouldn't code this way. I also use style mapping with W2x but contrary to you in Writer I usually create a specific style for bold italic text (both in Character styles and Paragraph styles). If you must report a bug or a feature request, you should ask LibreOffice developers to add a style by default for bold-italic text.