The preview in the builder endeavours to display the interface EXACTLY as it will render once deployed.
However the interface may load and render different content depending on the application it's being viewed in. Each application may have configuration options set including timezones, permissions and other custom data.
To emulate a particular application you can select an application in the emulation settings panel located at the top of the builder UI. Once you select an application the preview window will update and reflect how the interface will render with regards to the permissions and configuration of the selected application.
If you do not have an application selected, then the preview window will run in the context of you the user, so all content will be loaded and rendered through the lens of what you have permission to see.
Often you may find that you add an item of content to your interface and it won't load. For example you may add an image onto the screen and find it doesn't load. This is because the application that is being emulated does not have the required permissions to load and render that content.
To rectify this, you can either
Update the application so that it has permissions to load the content item
Update the content item to be in a scope that the application has access to, or make the item public
Usually the correct way to resolve this issue is to update the application's permissions