In this case BSEC (and MOx sensors in general) is not so much affected by the concentration ("heavily") of the pollution, as much as to which compounds are present and their effect on the raw sensor signal.
An offset in the gas resistance value is not a damage to the sensor nor directly affecting its sensitivity, but the BSEC library would take some time to self-calibrate. Clearing BSEC's state and restarting the library as you have done also does the job.
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