Not enough oxygen in the mix. If the mixture is correct, propane burns into mostly CO2 and H2O. If there is not enough oxygen, you get some CO2 and H2O, but also some C and CO. The C is seen as black soot on your kettle, and the CO is obviously a problem if you are in an enclosed area.
Check/adjust the shutter/damper/carb setting. Also check for obstructions (insects, spider webs, debris, etc.) in the air inlet or inside the burner, and check for plugged orifices.
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