A similar offer for 10% cashback on LNER appears very regularly on Halifax banking app.
It's advertised as a one time use only but I've activated it previously before purchasing a Plymouth-Folkestone ticket to take full advantage and in the following days until the offer expired, it also paid cashback on three other local journeys I made and booked on their app. That was in December to January and it's appeared on Halifax app again for February-March.
I've booked an off peak first class return Plymouth-Paddington for Thursday at £209-75p and the next day purchased a standard class version at £116 which I refunded shortly after, reason for it was I just needed a booking confirmation for a standard class journey as the hospital obviously won't refund a 1st class ticket. To my surprise, both journeys have now been credited in the cashback total in the Halifax app. Expect the latter will be deducted from the cashback after the refund is processed which I'm fine with is cashback definitely wasn't my rationale behind buying the second ticket that I refunded.
Curious who actually pays the cashback, the bank or LNER? Or split. The cashback on the first class ticket I purchased was £20-90p and can't imagine LNER would have made that much in commission by retailing a GWR journey.