This is a known bug that will be fixed in v1.0.6. We're very sorry about this annoying bug. For now you need to have at least 2 news sources for it not to revert back to the default. Instead of deleting the default sources, just uncheck them and add your custom sources. Sorry again for the inconvenience.
The sources are also not saved if you replace it with another panel. Originally this was not an issue because you could drag panels around to reorder. Since Apple had us remove that feature, it made less sense that we remove the saved sources when replacing panels. We will try to come up with a optimal solution with our current constraints, or at the very least save news sources app wide.