Each lender also has their own relationship profile with you. I got the iUP last year on a 256 7 Plus and had no issues, despite having taken a car on finance about the June before, and having been rejected by PayPal for an Apple Watch finance agreement worth £600 in July (probably because of the car).
There’s literally no pattern to getting accepted or declined, it depends on their relationship with you and their current business needs.
I’ve had and paid off 2 agreements with Barclays Partner Finance in the last few years, so I’d imagine my profile is strong with them.