We have the same problem on our Grande, has been like it for over a year now. It started whilst it was parked in an airport car park while we were on holiday. It was fine when we left and was bubbling when we got back. I gave up with FIAT UK and trading standards could not do anything at the time, even our dealership manager backed us up on the claim.
Unfortunatly, it does not fall under fit for purpose, because, the main functions is not affected (tried arguing this), it is only cosmetic. Might be worth having a discussion with the Citizens Advice Bureau, to see where you stand though.
They also seem to void your warrenty on the steering wheel if you wear a ring......
It seems that they have a way out because of wear and tear and contamination, from hand creams, petrol pumps, treating the leather/ or not. I tried for 6 months under our 3rd year, but then it fell outside the warrenty so gave up. They were also reporting that they had never seen it before, but when I commented on knowing people who have had theirs replaced then the story changed to we cant discuss other peoples cases.
The Grande supercede wheel is now more of a grey than black and more padded, but we refuse to pay nearly £400.00 to change it, so were looking at getting it recovered instead in better quality leather.
I have noticed that other makes of cars are suffering the same to, from the internet and personal experiences, so its not isolated to FIAT alone.
I wish you well and I hope that you might have more luck now theres more of the same happening.