Hello!
Thank you for making a bug report.
Based on the circumstances, I have determined that this is working as intended.
Imperion Magnum’s first line of text reads: “Must be Fusion Summoned with the above Fusion Materials and cannot be Special Summoned by other ways.”
This means that you must use exactly the cards named on the card for its summon. This is different from, say Sandwich, which requires 2 names listed on the card. In the case of Sandwich, because it does not have this text, you are able to fusion summon it using 1 correct material, and Swamp King to replace the other.
If you have any other questions, please feel free to ask in the #Rulings channel on the Duelist Unite Discord server and we can discuss this further.
If you come across any other interactions you believe to be a bug, please feel free to open up a bug report and we will look into the situation!
Thank you again for helping us make Omega the best it can be!