Not sure this is a bug, but I’ve run through all the tutorials and have just built a few nested repeating groups. There is only static text and an image in these, yet those elements don’t appear in the preview. I’m hesitant to invest a lot of time if the platform won’t handle a few nested repeating groups.
Here is a builder screenshot with the elements tree
A screenshot of the Properties panel for the RG’s might be more helpful; but at first glance, it appears as though the elements contain static data (as opposed to having used the dynamic expression builder). Also, check the Data source property for your RGs and make sure that there’s actually data being retrieved.