Source code is not available, and BME280 does not suffer from self-heating. In your case, the heating effect must come from the other components of the PCB. The easiest compensation method would be to let the board temperature stabilize, and record your personal offset. Note that this offset will only be valid after the temperature has reached equilibrium. Then you can use standard physics formula to convert relative humidity measurement from one temperature to another. In fact, the original post mentions 15C difference in temperature which seems excessive. I believe that BME680 is not the only component generating heat.
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