Celestial Fire is not working with the apostates blade set bonus. I tested it in the practice range by using the artifact mod that grants radiant on orb pickup and found the following: -There was no difference in overall damage or scorch applied when radiant vs not radiant. -While in Song of Flame, the set bonus DID apply and was the difference between a single melee causing an ignition or not (with the increased scorch fragment active). -Unpowered melees while radiant did apply the set bonus. I also tested it on prismatic with the arcane needle melee, and that did proc the bonus so it seems like it's only an issue on celestial fire. I did not test it on incinerator snap on prismatic or solar.