Yes I found the same. You can click the cross next to the "15% loyalty discount applied" message after you have logged in, and it removes the loyalty discount and gives you a box to insert an offer code, but it wouldn't accept "25OFF" when I tried, which is the discount code advertised on the main page.
I read a post on the forum here yesterday I think, from someone who had found the same, phoned FindMyPast customer service and managed to get the 25% discount applied.
Personally, after considering the amount of time I had spent trying to get it to work, and the time I might spend phoning CS, I just gave up and renewed at the 15% loyalty discount as usual. But the additional discount is around £18 on the Pro subscription, so it depends how badly you want it versus what you consider to be the time/cost benefit
