Samsung updated their theme feature on Android Pie to give designers and artists more creative options. The new notice in the Samsung Theme Store indicates that once there is an Android Pie update, the free themes will have a usable time limit of fourteen days. A pop-up will roll out and notify users once the theme is close to expiring and will revert back to the original theme upon reaching the time limit.
There’s no current clarification as to why Samsung made this decision and hopefully current updates will be explained soon as the One UI beta is around the corner.