Flame/Smoke not rendering in Eevee or Cycles

[omgold]

I believe it is because of the transparent background, which surprisingly keeps alpha=0 when encountering volume shaders one. See here for more explanation:

Cycles strange render behavior!

The problem is that transparent film adds an alpha channel which sets alpha=0 wherever the background can be seen. Your volume emission cube is still rendered, just the image viewer doesn’t display, due to the alpha channel. If you change in the image viewer in the top right drop down the viewing mode from ‘Color and Alpha’ to ‘Color’ you should see it again.