At 4 or 5 years old, especially with a lot of miles or a lot of weight, it's not unusual to experience spring flattening. I went through it with a 2007 Eagle TT. Even though the springs are rated for more than the load at max GVW, they will flatten. I learned a lot about this from a local spring specialty shop, and am now in the habit of replacing them every 4 years. I also replace the entire suspension components (bolts, bushings, shackle links, and equalizer at the same time) Rough roads and weight take a toll, then all you need to hit is a pothole or bridge joint just right and you'll be on the side of the highway with a broken spring.
From your description, you were lucky to notice it before a sprig broke.
2020 Eagle HT 30.5 CKTS Modern Farmhouse (16" wheels, spring shackles set on lower hole of frame, SumoSprings TSS-107-40, standard kingpin, B&W Companion slider, TST-507 TPMS)
2019 F250 SRW SuperCab 4X4 SB 6.2 (10,000# GVW) 3.73 Axle 18" wheels