Based on my support experience, the issue might be due to the following reasons:
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Access Through a Security Group: You may have granted access to a security group, and the user is a member of that group.
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Duplicate Reports: There could be two copies of the report, and the user has access to only one of them. Compare the report URLs—the one the user is opening and the one you have access to.
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Organization-Wide Sharing: While sharing the report, you may have selected the option "People in your organization can view this report.
I hope it will be helpful.
Thanks,