Some basic questions about an accelerometer's unexpected behavior while tilted
The vector magnitude is constant regardless of orientation, but you are applying offsets in one direction which throws off the symmetry.
You can't calibrate away gravity in only one orientation and expect it to work for all orientations. Thinking about what you did and if you flipped it upside down instead of a slight tilt. Would you expect it to be zero? It's adding all your offsets in the wrong direction.
Those gravity offsets are only useful if the accelerometer moves but orientation doesn't change. In that scenario, gravity is calibrated out because its direction and magnitude are always known.