Read permission is required on at least one master record to be able to see the solar panel records. Since Item is a junction object between Doc and Solar Panel, users need to have access to either Doc or Solar Panel to see Item records. If users have access to both Doc and Solar Panel, they can see all Item records related to either object. References: https://help.salesforce.com/s/articleView?id=sf.relationships_manytomany.htm&type=5