Hello Hieu Nguyen,
Thank you for reaching out to Microsoft Support!
According to your problem description, we ran the same test on this endpoint in Graph Explorer, but the test results are fine, and there are no errors. The test results are as follows:
According to your description, for some accounts may appear the wrong, please check that the assignment rights RoleAssignmentSchedule.Read.Directory are granted correctly and that the account has the following roles:
Reference document:
Hope this helps.
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