Steps to reproduce:
Obviously, it's unlikely that anyone would intentionally promote a search result referencing a document in a restricted place. As a super admin, however, I see everything and in this case did not notice that the useful resource I'd identified as the best result for a specific search term was in a private group.
The desired and expected outcome would be that a user without access to the promoted resource would simply not see that promoted result. Instead, she saw no search results at all for that term. She did discover that if she enclosed the search terms in quotation marks that she did get results.
From your description you might be running into an issue being tracked as either JIVE-60312 or JIVE-67737, however to confirm the behavior and receive updates on its status I would recommend submitting a case in your customer group here in the Jive Community as a next step.