SharePoint search results showing for those without permission to the documents

After nearly a month into a ticket with Microsoft, unfortunately I do not have an answer outside of reindexing the library and/or reindexing the site. The issue has resolved for us, but I never received an answer to why it was happening in the first place, or what exactly was done to resolve it. 

This was all done on a test library, on a test site, with very minimal content. Permissions were adjusted on the library, but only as per the instructions shared by Microsoft (see Customize permissions for a SharePoint list or library). 

My best guess is that indexing is necessary any time permissions are changed on a document library - but the lack of clarity on how to avoid this issue and the fact that it is a pretty massive security risk when dealing with any amount of personal information is concerning.