The key point here for Add Existing... when loading an html file for it to pull in any properly linked images and stylesheets the links to those images and stylesheets must be correct (work!) at the place you are loading it from with relative links (just like normal html). Otherwise Sigil can not find them. In other words if you instead loaded that html file in a browser before loading it in Sigil, all images and stylesheets must be found just like when importing an html file from an existing website or epub.
So based on Odamizu's post, the "problem" only happens when the links are already broken *before* attempting the AddExisting. Otherwise AddExisting works correctly.
So it happens when Sigil tries to fix up already broken links. Effectively it took a broken link and generated another broken link because it could not find the file linked to, or a resource by that name already existing.
Not sure if that can even be called a bug!
I will look into it when I get a free moment.
Vroni, if all you want to do is get the html without any attempt to get its linked in resources, simply copy it and paste it in. Or make the links in the html external ones (
http://blah) and they will not be touched.