I can't tell why this is: the box2d calls both engines make are pretty much identical, so I guess this must be something that changed in the 2.2 - > 2.3 box2d update. However it looks like there's a workaround: if you set the crossbar object (Sprite3 in this example) linear damping to 1, it acts more like a solid joint and still similarly to box2dweb. My best guess is the latest box2d version now takes in to account a body's linear damping when calculating joints.
Does setting the linear damping work around it for your game?