I have created and installed a preview app and have read some of the other comments here on this topic. I have added .article files to my Top Level collection and published. No update bubble shows up after closing and opening the app after a few minutes. Do I need to click the preview icon in the upper right corner of the Web Portal each time I want to preview changes on the device? I have tried that also, with no resulted update. If I delete the app and reinstall, my updated content then shows.
I do press the 'preflight' button after making content changes. However, the methods described by you (leaving the collection en/or backgrounding the app) did not work. What does work, is clearing the cache and data in the Preflight App on my device. I have to log back in, but that's no big deal. Then the update is available.
I can work with this method for now and I anxiously await the refresh method in the next release.