I summoned Jizukiru, the Star Destroying Kaiju (to the opponent's side of the field) and Cyber Dragon to fusion summon a Chimeratech Fortress dragon. Then I tried to use De-Fusion on Chimeratech and all it did was return it to the extra deck. Is this a bug or is it because of the nature of how Chimeratech is summoned?
It's because of how Chimeratech Fortress Dragon is Summoned. It's not a Fusion Summon, despite using Fusion Materials, so there were no "Fusion Material Monsters used for its Fusion Summon". Therefore, De-Fusion can't Summon anything when used on a Chimeratech Fortress Dragon.