Let's level set here...my tenant has NOT implemented the home site or the app bar.
When we visit the SharePoint Start Page and search anything at all (things that we know actually exist like the word, "the") no results are shown on the "All" tab. If we switch to the files tab, we can see some results.
I have tested this on another tenant that does have a homesite and the app bar - when visiting the SharePoint Start Page - search results do show up under the "All" tab.
Has anyone else experienced this in SharePoint Online recently?