any oils/grease in that area will smoke off once heated up. There was actually a flashlight I remember somebody talking about that would smoke up and fog up the lens. Run the light without the lens and reflector and let it clear. As long as you aren’t smoking the LED, it is likely oil/grease heating up or any silicone in the thermal adhesive. You just need to determine if its the dome smoking and if it’s overdriven too much. IMO, Bad contact/thermal interface between the star and heatsink/pill would be the first place to look for causing the LED to smoke. Overdriven is #2. Other source would be grease, silicone, moisture, etc I mentioned above.
edit: Here is the thread about the same thing