Its not something iv ever come across to be honest and I spend a bit of time on a 5 series forum I know its very common for the rear e46 springs to snap , you can get e39 eibach kits delivered from german ebay for around 170e all in cheapest iv seen them anywere for a 5 series

Was the 528 an original sport ,was the shock original ,your other shock could be shot while the other from the 528 perfect or vice versa ,plenty of factors to consider thats why its recommend to replace in pairs