Report Displays a Hidden Field to Unintended Users

The field is hidden only for the Page Layout and not for Field Level Security

Please be aware that when the field is hidden because of page layout the users can still access the field via reports, search results, list views, and the API.

To confirm this, 

1. Review the field's Field Level Security settings.
2. Note that the field is set to 'Hidden' for some Profiles.
3. Hover over 'Hidden' and either field is hidden because of page layout or field is hidden because of Field Level Security appear.

• When the field is hidden because of a page layout, the users with the corresponding profile have access to the field via reports.

In order to correct this, Click Hidden for the correspondent profile and deselect the Visible checkbox.
 

The field is actually another field that has been renamed

• If the report has been created using a Custom Report Type, the field may have been renamed via 'Edit Layout' section. For further information, see Design the Field Layout for Reports Created From Your Custom Report Type.
• Translation Workbench users - Review if there is any translation for the user's language on a field in the same Object. For further information, see Manage Your Translations and Translate Metadata Labels.
• Managed Packages - Review if any field has been overridden. For further information, see Override Translations in Second-Generation Managed Packages and Unlocked Packages.

Field Level Security has been overridden by the User's Permission Set

For further information, please see Set Field Permissions in Permission Sets and Profiles.