@TechUST, Thanks for posting in Q&A. From your description, it seems Guest account is unable to sync files on Autopilot enrolled device. But it works on non-Autopilot devices. Before going on, could you confirm if the non-Autopiloted device is not enrolled into Intune either?
If yes, I wonder if any policy related with OneDrive or SharePoint may affect it. To narrow down the issue, please go to Intune portal and find the affected device, check all the configuration policies applied to this device to see if any OneDrive or SharePoint policy applied to it:
Choose one test machine without these policies applied. Add one by one to narrow down which one may affect.
Please try the above suggestion and if there's any update, feel free to let us know.
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