Is it possible that the plate is clamped in fractionally off centre? There might be just enough slack in the 2 holes to allow a little misalignment. Yes I know it's a pain to have to slacken the screws again. Perhaps you need to need to have the butterfly fully, hard shut, before snugging up the screws initially and then doing the final tighten and lock when you're happy it fully closes the throat. Maybe Rhys will comment if he reads this.