Smoke refuses to show up in exported renders

Help, I have no idea what’s happening here!

First image is from the Render View in Blender. Second image is a PNG with Alpha saved manually from the Render View window.

The smoke also disappears from exported frames when I Render Animation (I’m trying to render just the smoke as a separate pass on a transparent background).

If anyone has an idea about what’s causing this I would really appreciate some assistance. Thanks!