From your description, it seems those device use the MAM without device enrollment, or MAM-WE to access corporate resources. For such kind of MAM, it allows IT administrators to manage apps using MAM and app protection policies on devices not enrolled with Intune MDM. So when you use app protection policies, please make sure that the office suite apps have been added to Allowed apps blade, and there is no restriction settings to corporate data in Advance settings part.
Here is a sample for MAM-WE with app protection policy, just for your reference.
https://www.petervanderwoude.nl/post/windows-10-mam-we-and-office-desktop-apps/
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