The car has ~600 lb-in springs in back, with a .5:1 ish motion ratio. Even if you just take it 1:1, 50lb of weight loss = 1/12th of an inch more ride height, or 2mm. Unless your car is freaking out every time you put something in or take something out of the trunk, that's not it.
It sounds like the SZL signal just got freaked out and after driving it is working within range and the computer decided it's not actually broken