Internet search watch lists support poster OR subject filtering on the search side. Headers support doing both at once because headers supply both at the same time.
You can make it work the way you want by making it a "Poster Search" only then making and assigning a filter profile to the watch and add the subjects you want to this filter profile. The way this works is:
1 - Search returns all results from the poster.
2 - Filter profile filters out or in the subjects you want.
You can also go the other direction too, search for a subject in the main search then use a filter profile to filter down to just the poster you like. Typically I think the first way works better. Return all results from poster, then filter in or out the subjects you want.