I had a 50% relapse on my class 3 open bite bsso and lf1. In my problem it was that there was too much movement attempted through orthodontics, and unless you have incredible maxillary/mandibular bone, too much orthodontic movement will predispose to relapse or gum recession (which I also have mildly).
In my case the relapse was evident within months of taking off the braces. It stabilized within a year.
Probably most late phase relapse like what you describe is orthodontically mediated.
If you have an open bite you probably had a narrow upper jaw like me with possibly some crossbite like I had too. If that is the case, surgical arch widening via multipiece lf1 or sarpe are the only "ideal" ways to correct the problem.
Braces are only good for fine alignment. Gross misalignments must be corrected through bone cutting and repositioning.
Bones heal within a month or two so any relapse after that is not likely bone related unless you develop a subluxation of your TMJ or condylar resorption.
It's amazing how many orthodontists and surgeons don't "get" this. I went to the surgeon who was renowned as the best in my city (Tocchio) and I still ended up in this position.
