So . . . it's definitely behaving more predictably now. Thank you!
As a follow-up, I tried to create a new result source with a query of simply "{searchTerms}"--and nothing more, in the hopes that this new "broad" result source query would include results from my two new content sources. But the new Result Source ("Everything") does not appear in the Result Source drop-down list in the Search Tool. Is there anything special I have to do to ensure that the new Result Source is populated in that drop-down? (Also, if I set "Everything" as the default result source, I am not returning any of the results I expect from the new content sources either...)
As a follow-up, I tried to create a new result source with a query of simply "{searchTerms}"--and nothing more, in the hopes that this new "broad" result source query would include results from my two new content sources. But the new Result Source ("Everything") does not appear in the Result Source drop-down list in the Search Tool. Is there anything special I have to do to ensure that the new Result Source is populated in that drop-down? (Also, if I set "Everything" as the default result source, I am not returning any of the results I expect from the new content sources either...)