This is honestly a strange problem to have. Because most 135i owners get pissed because a simple gallon of milk sets off the sensors. I know mine does. You may have to have the sensors recalibrated. I know thats something that likely the dealer would never check, but if they were calibrated in correctly at the factor (meaning likely someone forgot to add the weights when they calibrated them.) this could happen. Might wanna ask your dealer if they can check the calibration of the seat module itself and see how much the seat reads with someone sitting in it and compare that to the weight the module sees.
Honestly if this issue does happen, its unexceptable and BMW should fix it for you, its a safety risk which costs nothing to fix, even if they have to dumb down the sensor.