I've found similar results. I grow perp with 4 plants per 2 week rotation so I have ample chances to try this. So far veg defoliation slows vertical growth but builds roots during that time, and early flower defoliation gives terrible results. i lightly defol'd a plant the first week and it's on week 5 now and has half the weight on it that its untouched sister has
Definitely a habit to start in veg, I suspect the combo shock of flower + defoliation throws it into panic mode.
Never defoliate before the stretch is over in flower. Even if you defoliate in Veg, do NOT defoliate until after week 2 flower. I wait until day 20ish Flower, and nothing will be stunted.