It may be an issue with the cache on the device that is running the app. On a computer browser, I know how to force a refresh. But on a phone or tablet I don't know how. But I think that's what you have to do. I don't know if will be as simple as closing the app completely and starting it up again.
If that doesn't work... I think in the App, there's a way to switch over to the WebUI. Do that and see if the Playlist items update correctly. If so, then switch back to the previous view.
Hopefully, someone will come along with a more definitive and helpful answer, and I can delete this post. ;o)