I think there might be a problem with IK override and blending animation.
Without blending, override is working fine (although from time to time it seems to "jiggle" a bit but maybe that's my setup ?), but with blending, the override bones are like they dont know what to do until the transition is over.
I've made an example in C3 (R225 stable) if you want to take a look:
It's also happening with "override Object" when setting X or Y the object (head in my example) moves away during the blending.
Not sure if it's me doing it the wrong way or a problem with the plugin ?