My practice heads fly exactly with my field points to any range I would care to shoot.........

Did you try the practice head on several different arrows and shoot it multiple times or just one or 2 on a single arrow?
Try it some more mixed in on different arrows and in the middle of 3-6shot groups.

I've seen where people will shoot 2 field point arrows and always shoot the broadhead last and wind up peeking or dropping their arm because they want to see where it goes and they shoot it differently.