If your AHC system is still functioning properly, the rear is most likely low due to a mis-adjusted sensor.
How is the health of your AHC?
Does it go up/down properly?
Do you have Techstream to measure your AHC pressures in N mode?
Have you parked on level ground and cycled from L to H, while counting the graduations (tic marks) on the AHC fluid reservoir? 10 or more graduations change are what you're looking for.
What is your height from fender lip to center of hub at the front/rear? Height measurements should be 19.75" front and 20.5" rear.
And I forgot to actually answer your questions.
1) the springs do weaken but they do not affect the height of the rear, only the hydraulic pressure in the shocks
2) many people have reported needing new springs AND 30mm spring spacers to bring the pressure back down to within spec, even at factory ride height and factory weight