To me this is the critical issue and players like @AeonNhiyr should be provided some sort of relief. The developers chose to name them according to existing sets in the game. Maybe they did this because of theme. Perhaps they did this because the anticipated the possibility of adding additional items to sets along the way. Who knows why they did this but the choice at the outset should have been to name them just like the other two DLC items, making them completely independent and not subject to any confusion about association. It’s entirely reasonable that a customer would buy the Serpentine Flame Ring expecting it to be part of the Serpentine set. Why wouldn’t you think that? The developer made a mistake here for whatever reason and its not unreasonable to ask them to at least provide some offset since they ultimately decided that they were not going to make them part of the set despite an abundance of evidence that it would not imbalance the game in any way, but instead makes the buyer worse off because they are less able to make a set like other players!.
IP2 and 505Games should do the right thing and provide some form of recompense for players, especially ones that have been as patient in learning the resolution of this issue and continued to be paying customers like @AeonNhiyr