It should correct itself on the next billing cycle with a pro-rata refund, check your bill for your billing date, not when the payment is due but the date the bill was produced, then check your order confirmation for when the new discount starts.
I'd guess your new order was open and in progress when the billing cycle ran, parts had completed and parts in progress, which is why the bill is messy, it will correct itself next month, it's just unfortunate timing of the bill being generated at the same time changes were being made.
