Searching on a SharePoint subsite not working for external users.

Hello

We have a portal for collaboration between people working here and customers. This is a SharePoint site with each collaboration space created as a subsite on that site. Each subsite has its own permission groups etc. The issue we're facing, which has been coming and going, is that the search is not working for external users.

When trying to debug this I have given an external test account every combination of permissions I could think of, but to no avail. Users who have reported this error report that they can navigate to the files they want to search for, but when entering things in the search bar there are no results.

Another strange quirk is that the search field is in different locations for external and internal users. Internal users get their search bar on top, while external users get their search bar above the site navigation

Any suggestions? What are some settings that could affect this?