Rising Tempest: Rapid Shot does not proc it

I equipped two skills: Rapid Shot and Oil Grenade. When I use Rapid Shot, the DPS value remains unchanged, regardless of the skill Rising Tempest supports. When I use Oil Grenade, the DPS value of the supported skill goes up, as expected. Even if it is supporting Oil Grenade.

Rapid Shot is tagged Fire, so I expect it should behave as an Element-tagged skill.

Further, if anyone reads these, Rapid Shot 6% Gain as Fire per Heat does not appear to function on Grenades. The first Heat value shows on the DPS tooltip, but subsequent heat values do not. At 28 Heat, I expect to see a substantial difference in the effect of Oil Grenade. Most of the time I do not. The times I do see a difference, I suspect all the damage is actually a Herald of Thunder/Ice interaction, not a Heat-buffed Oil Grenade.