Are you 100% sure a spring hasn't broken right near the end? The coil springs set the basic ride height, the SLS system adds refinement and can raise the back to compensate for heavy loads but it can't lower it as such... gravity does that when the SLS valve moves to the discharge position allowing some pressure in the struts to bleed off. The system doesn't independantly level each side, the SLS valve is 'driven' via a linkage on the antiroll bar and the valve simply diverts hydraulic oil either to the struts (when the back is low) or back to the resevoir when the back is high [/gross generalisation]
When a sphere dies/all the nitrogen excapes the space previously occupied by a compressible gas is filled with incompressible oil. The level in the resevoir drops, the ride gets hard and bouncy (both of these usually happen gradually) and IME (on 123s and 124s) there's either no change in ride height or the back ends up a bit higher because the gas spring has been replaced by incompressible oil i.e. there's less compliance in the suspension
