I have recently started using a BME680 and I do operate it in the office. The behavior, respectively the measured values seem a bit strange to me, see screenshot in the attachment.
Around 11:20 I opened all windows completely for about 5 minutes, so that there is a draft.
Immediately before venting, IAQ was at 113 and eCO2 was at 660 ppm. During ventilation, both values drop as expected.
Then, about 30 minutes after opening the windows, the IAQ is 265 and eCO2 2600 ppm. So, within a short period of time, the values have increased dramatically and continue to remain significantly higher than before ventilation.
I would actually expect IAQ and eCO2 to drop after ventilation and remain well below pre-ventilation levels for a period of time and then gradually rise again.
So if I were to rely on the data, the air quality would be much better if I refrained from ventilating. That seems pretty implausible to me.
Apart from the airing, the conditions in the premises have not changed.
eCO2 output from BSEC library is derived from IAQ output value. So they are related. If the air from venting contains some other VOCs caused by paint, motor oil, etc., then IAQ will become worse after venting which will cause eCO2 to get worse.
If possible, please provide the data log from your tests.