This topic has been reported multiple times but I do not see it as a known issue.
Subracers exotic perk is not activating properly with celestial fire. When using incinerator snap, the scorch it applies will activate Sunbracers (i.e. the enemy survives the initial hit but scorch kills them). With celestial fire, if the enemy does not die from the actual melee hit, Sunbracers will not activate from the enemy dying to scorch.
Bungie please acknowledge this issue!
-
Use the finger snap. It reliably activates Sunbracers, and you’re more likely to get a kill with it.