What causes fire to gradually turn off?

I use blender 2.80 (The old smoke simulation). When I bake the last 251 frames of the movie, the fire works well then, it gradually turns off!

What could be the reasons that causes fire to gradually turn off? And how to solve this?

I attached the file here: Crazy smoker.blend (2.5 MB)

Here are the steps to reproduce the issue:
1. Bake.
2. If you navigate before frame 9039 you will find the fire in the rendered image.
3. Around frame 9039, the fire gradually disappears or turns off.
4. At frame 9060, the fire completely disappears in the rendered image.

I need the fire to last until frame 9172 so the fire should be present in this range (8922-9172) without getting weak, disappearing, or turning off.

Thanks in advance,

Okay. I already knew the answer. I adjusted volumetric settings.