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Dear Jared Jarvis,
Good day! Thank you for posting your query in our community.
Based on your description, a specific user who has the right license and permission can't access shared files in SharePoint. And you have tried some methods, the problem persists.
Given this situation, we assume that the problem may be related to the backend, more resources need to be involved on the issue in your environment. I suggest you follow the steps in the article to open a ticket, this is the most efficient way to fix the issue in your environment. Because the engineers in related team have higher permission and more resource than us to help check it from background and collect the log remotely, and they will continue to investigate this issue further and provide suggestion for you.
Appreciate your understanding.
Sincerely,
Jazlyn | Microsoft Community Moderator