"All sources" filter

We've set up several graph connectors in the search center and plan to add more in the near future as we move towards using M365 search as our enterprise search solution. During our testing, we've noticed that the "All" search vertical sometimes shows "All sources" (a drop-down list of all of available content sources) and sometimes the "All sources" filter isn't available. We haven't been able to identify a pattern -- sometimes "All sources" will show up for a particular person one day and not be available the next day. We'd like to make the "All sources" filter available for everyone and have it show up consistently. Does anyone know what determines when the "All sources" filter shows up?