Inconsistent result from Azure CLI between Service principal login and user login

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I wanted to programmatically retrieve role assignment, and I used service principal to authenticate to run the following command:

_az role assignment list --all --include-inherited --subscription xxx --query '[].{principalName:principalName, roleDefinitionName:roleDefinitionName, scope:scope}'_

However, principalName always shown None. I tried az login with my user account, and it shows PrincipalName. Is there any missing RBAC required for my service principal or is this a bug?

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#### Document Details

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* ID: 9dacfad8-cb2f-2534-3b29-89b0930a2252
* Version Independent ID: 17325cd4-44c5-4069-d9f6-940a55f0f7c9
* Content: [az role assignment](https://docs.microsoft.com/en-us/cli/azure/role/assignment?view=azure-cli-latest#az-role-assignment-list)
* Content Source: [src/azure-cli/azure/cli/command_modules/role/_help.py](https://github.com/Azure/azure-cli/blob/dev/src/azure-cli/azure/cli/command_modules/role/_help.py)
* Service: **azure-resource-manager**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**