@JoonieB - indeed, any series which contains a book which has a series number > 999 will exclude the whole series from series checks. In theory I could make that threshold configurable for a future release.
However as you say it is a highly rare exceptional case, and the downside of producing thousands of lines of logging and performance impact of doing so means it isn't something that most would want. In my own library when those sort of nonsensical series numbers come up I just remove the series information completely. It is clearly just some made up "series" denoting an order of publication, with no meaningful relationship between the books. When there is a relationship between them (a mini series), Goodreads tends to list that instead. And as the metadata download plugin can only retrive information for one series even if there were multiple on the web page, you would have to manually assign and overwrite series names and values if you chose to prefer the high numbered "publication" series over the more meaningful content based ones.
So just delete the "Silhouette..." series name info and don't worry about it would be my advice
