In recent years, worldwide privacy laws have led to several changes in Facebook policies and practices. For instance, users must reauthorize permissions every 90 days for apps that interact with Facebook, such as DoubleDown Casino.
When app permissions are not reauthorized in time, Facebook will remove the now-inactive user from that app. If it has been more than 90 days since a Facebook friend has been active in the casino, they will no longer appear in your Gifting tool or contribute to your Daily Wheel Friends Bonus, so they will need to log back in through Facebook and reauthorize permissions.
Facebook has published a blog post explaining this update to their processes: https://about.fb.com/news/2020/07/improving-data-limits-simplifying-terms/