[jjd-uk]

It sounds like the name of the setting (or the behavior) is backwards, based on your description (and what manfeed says happens with it checked).  You say it's off by default, so that it won't scan extras into the library.  This sounds like it is "Ignor(ing) video extras on scan" when it is unchecked, ie disabled.  Enabling then does add to the library, as in your last statement...but that's the opposite of "ignore".

This is because you're not understanding what I wrote. Although enabling does add the Extra to the library, it will add it as a movie so it's a false positive which you would not want. To try and make it clear, you have for example an extra call "Making of ABC" since it will be in a subfolder of the movie which is within the movie source, the filename will be pass to the selected movie scraper and the scraper will try to indentify it as a movie, if the metedata site (e.g. TMDB) returns a result then the Extra will be added as that movie which is false, so the title you see in the library will not be correct, hence a false positive.