When reports are built on objects that do not support sharing settings the field visibility in the report in the experience site differs based on the type of report.
Example: If the Sharing settings for Opportunity are set to Private, Quote is Controlled By Parent and for Quote Line Item - not supported in Sharing Settings.
Standard reports (Quote with Quote Line Item and Quote PDFs): The fields from the Quote Line Item will be visible in the report for external users.
Custom report (Quote with Quote Line Item): The fields from the Quote Line Item will not be visible in the report for external users.
As per this: Allow External Users to Create and Edit Reports
It is mentioned that
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