It could have been a Sling firmware or player update that changed it. Those remote skins are just ascetics, its more important that the buttons function as labeled. Should be glad that you didn't end up having to change to remote with a generic skin.
I had a similar problem a long time ago with the SA 8300HDC. One day it was working fine. Then after one of Slings updates, the "List" button stopped working. Luckily someone figured out that the "Set" button on the Sling virtual remote did what the "List" button should have done. There were other quirky responses, so I loaded someone's custom remote file and it worked fine.
So, if it matters to you, you can try out one of the remote files from one of the links blows that match your box.
The next thing you should do is open a new topic in the Enchancement Forums, letting them know they botched another remote file and to fix it.
Right, I'm guessing here, but I think it might be a change in Slingbox. The original Classic, the Solo and the Pro were all country-dependant, one design worked in the USA and one worked everywhere else (Often known as PL & RV after the type of IR chip they used) . In those days the remotes and their 'skins' were all stored locally on your PC and were dependant on the Slingbox design. The Non-USA 'RV' ones had the Sky and Sky+ 'skins' included but the US ones did not.
The next stage was around Slingplayer V2.0.3 when Sling decided to store the remotes and 'skins' on their server and just download the appropriate ones to your PC when you ran the Video Setup. In theory that was supposed to speed up the production of supported devices. However, they still (unnecessarily in my view) maintained two different databases for the two different models.
Then the Pro HD came out and was initially released only in the USA, and still used the USA database. Then it was released in the rest of the world, but for the first time they did not produce different models and it was US-version that was released. I can only assume that they had to produce a new database that contained all the devices from the 2 databases. So maybe they made a bit of a mess with this and didn't take the Sky+HD box across?
And the reason no-one else has reported it is that the Pro-HD is relatively new outside the USA so most people using the Sky+HD box (like me) are using a Classic or a Solo, which are still accessing the non-USA database.
As eferz says, you can count yourself lucky you at least have a remote that looks a bit like a Sky Remote. I happen to have US Slingboxes (they were much cheaper) and had to build my own remote with the generic skin.
(Although I think this makes sense it might very well be absolute rubbish )