The same happened to me, but it seems like guys from SQUAD just changed the 'connection' simulation. Even if I try to do that with stock parts, this issue happens, mostly because your connected stages are more powerful than the one they are connected to and so they tend to sort of rip off the spaceplane... I guess that one possible solution could be to increase strength of the radial decouplers, so that they would be able to sustain that much of a power instability... for now, you can try attaching more struts between these stages... that might possibly help...