Scenario :
This scenario can occur because the role of the Account Owner has the Case Access and Opportunity Access set to "Users in this role can view all cases/opportunities associated with accounts that they own, regardless of who owns the cases/opportunities ." In order for User AAA to be able to add Account Team Members with read/write access to Cases and Opportunities, the role of the Account Owner would have to have the Case Access and Opportunity Access updated to "Users in this role can edit all cases/opportunities associated with accounts that they own, regardless of who owns the cases/opportunities."
If User AAA is a standard system administrator or has the 'Modify All Data' profile permission, then the above explanation does not apply.
See also :
000384833

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