Interaction between damage gain and damage conversion issue.

I've play rapid shot skill with acolyte of chayula.

Concept of this build is add flat elemental damage and change them to chaos damage by equipped original sin ring and scaling them by cluster of chaos passive.
I choose rapid shot because its can do more hit to do more roll a reality rending (ascendency passive).

So every damage output should be chaos damage by original sins ring but when i used rapid shot i found out the monster that hit was ignited ? (I thought that from Rapid Shots Gain 7% of Damage as extra Fire Damage per Heat)

My question is If damage gain do the same mechanic like damage conversion why damage gain can't convert ?