Well then there is a bug with hardcoded custom post types.
In the Realm pannel -> Object Scope, I can see my hardcoded custom post types in the list of available items.
But if i check the checkbox in front of one of them and save, my selection is not saved.
I did the test with a custom post types created with Custom post types UI and it’s working.
Any hint ?
Don’t hesitate to ask if you want more infos to debug ??