A user who is not defined as an Organizational Unit manager can be assigned to the Managing My Area work center. Within Managing My Area such user has access to data of all Org Units (comparable to a user who is manager of the highest Org Unit).
This is the expected system behaviour.
The Administrator has to ensure that the Managing My Area work center is only assigned to users with manager function.
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SAP BUSINESS BYDESIGN 1708 ; SAP Business ByDesign 1711