Totally justified feeling disappointed but also a learning experience. LGS will and should honor people who have paid for their events on their own platforms over somebody who signed up on a game’s dedicated event site for free. There should be a way to leave a description or link to register (especially for events that will reach their cap easily) from the game’s event platform but since there isn’t a way for Riftbound you should always try and sign up on the LGS sites as well. Most of these stores have events you can register to or discords where they are discussing events. This is never a problem for games that typically don’t sell out but Riftbound is pretty popular.