Not sure if this is just a UK thing, of if US customers will be affected too, but it appears that Amazon have dropped their Prime discount for any pre-orders:
I'm still salty that we only ever got a £2 whilst US customers got 20% (I think).
We would now like to inform you that we will no longer offer the Prime discount of £2 on physical video game pre-orders from 7 August, 2018.
I'm still salty that we only ever got a £2 whilst US customers got 20% (I think).