Do you mean 40 to 90pa or 40 to 90hpa?
Could you tell us in what environment the senosr was used? include temperature and pressure and other conditions.
Do you tell us what did you do in two months? Do you power off and power on BMP280 or alway keep BMP280 running?
Please offer more information to us.
It is better to offer the entire sensor data from two months ago to now.
Looking forward to your further inforation.