When I started taking it apart to see if it was fixable I found that the entire damn thing was built out of stapled-together 1/2" OSB (oriented strand board...garbage "plywood") and all of the cuts had awful stress concentration corners which caused the frame to pretty much disintegrate. Wasn't a real screw or nail anywhere in the piece that I could find.
I spent a day rebuilding the frame with about $30 worth of construction lumber (some 2x4s & 2x8s) and hanger brackets. It's been perfect ever since.
Completely inexcusable that a $30 aftermarket fix is all it took to dramatically improve an $1800 sofa.
Edit: I found a few images of the damage and a couple of the replacement supports I added. There was a lot more, but at the end of the day I was tired and didn't take a ton of pictures https://imgur.com/a/bqlLgW3