System Information
Windows 10 x64, 2 x GTX970

Blender Version
2.79 5bd8ac9

Short description of error
If the 'flow type' is set to 'fire' in the Flow object's smoke tab, the resulting alpha is much more transparent compared to when the 'flow type' is set to 'Fire + Smoke'.
The alpha/render looks correct with either 'flow type' setting however, when viewing the 'rendered' mode in the viewport.

Exact steps for others to reproduce the error
Change between 'fire' and 'fire + smoke' settings and simulate & render with transparent selected in the 'film'section of the render tab.
Sounds like an old bug from my googling; just wondering if was still in hand to be fixed?