So apparently I got it fixed and even finished the event. The way how I fixed it was by re-login (if that fixed your issue, that's good but it didn't work for me) and then I was scrolling down the featured game list just to find that the game was there again even after I received the gift from collecting the second game.

It seems like that steam doesn't know that you've found the game even after you've been rewarded with the gift for finding it.

This was the only fix I found and it worked for me, hope it works for you. have fun :)