I am monitoring the acceleromter and gyro static offsets as I'd like to know if they change throughout the sensor lifetime. I see significant changes after a couple of months even though the harwdware hasn't changed. Is this expected? If yes, what can explain these changes?
You could see accel's zero-g offset and gyro's zero-rate offset over life time in BMI270 data sheet. If your sensor's offset were out of range in data sheet, you could refer BMI270 HSMI document https://www.bosch-sensortec.com/media/boschsensortec/downloads/handling_soldering_mounting_instructi... and check whether your use meets this specification?