Rapid Fire's Heat and Mortar Cannon / Mirage Archer

I noticed that Mortar Cannon and Mirage Archer, if slotted with Explosive or Cluster Grenade, will have their tooltip be updated to reflect the increased damage they would have done with consuming heat. But they don't consume any heat while firing. My question is, is this a tooltip bug? They aren't getting any benefits from heat right since they aren't being consumed? In which case, the tooltip is lying?