Soot or abnormal combustion signs need appropriate gas-safety assessment. They should not be treated as a routine cleaning inconvenience.
What the observation means
Location and timing matter, but an owner should not adjust a burner to see whether the deposit disappears. An apparently normal cooking temperature does not establish safe combustion.
A useful next step
Stop using the affected appliance and contact a suitably qualified gas professional. If gas or fumes create an immediate concern, follow emergency guidance. Retain the observation for assessment and do not clean away all evidence before describing it, or continue cooking to confirm the pattern.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.