I was having the same issue, using this addon: https://simonthommes.gumroad.com/l/BRNbl
Render result:
Save settings:
Saved image in windows explorer:
I was seeing unexpected results when previewing the alpha channel of my render result:
While checking out the shader I noticed the principled volume shader density socket was unused and set on 0. I’m guessing cycles ignores emissive volumetrics for generating alphas. Once I plugged in the value I was using for the emmision strength input into the density input, cycles was able to generate a proper alpha result.
Since altering the density value changed the look of the fire quite drastically, doing the following in the compositor allowed me to keep the look I want, but export with transparency. Bit of a dirty fix but works well enough.
exported image:
