The theory is that you can't directly add groups to ssrs permissions once the ssrs instance has been reconfigured by SCOM.
You need to create a SCOM role, add the AD group to that role and then add the role's guid to ssrs permissions.
Read more here : https://myitworld.azurewebsites.net/2017/01/09/manage-report-operator-user-role-scom-2016/
However, there have been reports of ongoing issues with granting access to ssrs for quite a while now : https://myitworld.azurewebsites.net/2017/01/09/manage-report-operator-user-role-scom-2016/