I agree that the behavior is misleading where you type a price in the order window, and press enter, and a different price is used.
I suggest that in this case, pressing enter does not submit the order, but instead it: updates the price box, blinks the price box, has a tooltip available explaining the new limitation on hover.
In the case where the price passes the validation, pressing enter submits the order.
So this poster would have to press enter twice, but imo the visual feedback would be worth it.