but did not describe it as a flaw, rather as something that all the hunters do
An overarched back or duck butt is never correct. If you look at photos of the top hunter seat equitation riders and Medal Maclay winners, you won't see many duck butts.
In my area of Virginia, you won't see it much in local shows either, it's penalized as a form fault. You might see an otherwise effective rider pin in spite of it, but you'll never see a rider pin because. I can't speak for other areas of the country and other show circuits.
One of the ways the disciplines really differ is that a round back, or breaking at the lower back, is fairly common in eventers and jumper riders, but is considered a very serious form flaw in hunter seat eq.
Without photos from the OP, I'm going to continue to assume that her instructor is trying to correct a round back, and get to a more correct, functional positions. If that's not the case, I can only assume that on their local show circuit it's judged differently, in which case I'd advise the OP to get some equitation text books and some magazine subscriptions and aim for higher than that local circuit.