This article describes the behavior of Dynamic Address Group in FortiManager.
When adding a new object in the address group and the address group is being used in active policies, the expected behavior is the policy package will change status to 'Modified' and in install preview will be seeing the expected changes.
This is applicable if the address group is static and not dynamic.
If the address group is dynamic, where it is mapped to per-device mapping on managed devices, it will need to add the new object to the Members in Mapped Device.
After adding the new object in mapped device members, then the policy package status will be changed to 'Modified' and expected changes are showing in Install Preview.