Views "Relevance" sort criterion not sorting by relevance

I have Search API installed, using Solr. I created a view with an exposed Fulltext Search filter [see attached]. This works fine: search results are returned according to relevance to the search term.

However, when I add a Relevance sort [see attached], the results are no longer ordered according to relevance but rather just filtered by the search term, not ordered according to relevance.

Isn't this opposite of what should actually be happening? Adding the Fulltext Search filter should merely filter out content that does not contain the search term, whereas adding the Relevance sort is what should further sort the results according to relevancy.

The reason this is such a problem is because I cannot set a default fallback sort order [see attached] for when there is no search term (no search term would return all results, each with relevance of 1, therefore falling back to a second criterion, see "Authored On" in attached).