How is this 50Hz noise coupled to the sensor and why does it disappear when wired in a particular way?
It could be a Faraday shielding. In a car, the mobile signal is reduced even with big openings/windows. The car is insulated from earth via tires.  The shielding of the load sensor and the shielding of the sensor cable can reduce the noise like in a Faraday cage, even without any connection to earth. In the contrary, sometimes the noise gets stronger if this shielding is connected to earth, since there are still more connections to earth resulting in a loop that acts like an antenna for noise. So sometimes it's important for shielding to have only one connection to earth at one single point.