Closing as by design.
Previously the 'loader layout' option (which your .capx has set to "Yes") was ignored in preview mode, and only had effect after exporting. The latest betas enable loader layouts in preview mode. A limitation of loader layouts is that you cannot create any objects from other layouts until the loading has finished (reached 100% progress) - otherwise it would have to load the entire project to display the loader layout, defeating the purpose of the loader layout! In 'Start of layout' on a loader layout, it is certain that nothing from other layouts will have loaded. Therefore the event will not be able to create the object. In other words, the bug is in your project, and it would previously have only shown up after export, but now it shows up in preview mode too (which is useful, it helps you catch the problem sooner).