Ah I see, yeah now I see how hectic it is. Its like the boobs are at war with each other for the chest. Its pretty laughable on my characters, hilarious in fact.
I was able to get stable settings without substepping, and for me it was the setting Soft Constraint in the Angular Limit that helped with a lot of the jitter. I think the 2 settings Stiffness and Damping there are a proportion of some kind, so even if the Damping is at 0, if the Stiffness is low it will still let Damping through. Too high of Damping makes the physics body drag a lot and flop a lot, (if its boobs they will turn into flapjacks), and too high of Stiffness makes them jitter. If you wanted to try and get passable physics without needing substepping, at least until its fixed, here are my settings I used for boob physics that can be applied to any constraint really, perhaps my settings can help for now
