So in the first iteration of this post I seem to have found why the widget doesn’t work: Valve recently made Steam Store links more readable by putting the name of the game behing the AppID which turns
http://store.steampowered.com/app/394690/
into
http://store.steampowered.com/app/394690/Tower_Unite/
In the preview at least the text works, made posting Free Game Threads just a little bit more annoying than it needed to be.
The code/RegEx seems outdated in the way that the old URLs still work, the added game name screws it up though.
With game name:
http://store.steampowered.com/app/394690/Tower_Unite/
Old URL syntax:
http://store.steampowered.com/app/394690/
TL;DR: Until the code is updated, remove game name from URL to make Steam Widget work.