It returns an array of permission scopes by default. If you want to export all attribute values of the array, then you need to convert the array to a JSON type string.
For example:
Get-MgServicePrincipal -Filter "tags/any(t:t eq 'WindowsAzureActiveDirectoryIntegratedApp')" | Select-Object -Property id,@{Name='oauth2PermissionScopes';Expression={$_.oauth2PermissionScopes | ConvertTo-Json -Compress}} | Export-Csv -Path "D:\test.csv"
Hope this helps.
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