Now what? Is it supposed to pop up in my library? Because it didn't. How do I claim it?

Thanks for the answer and solution.

One has to wonder, though, why GOG still offers this product when they know it's not working properly. You'd expect them to remove it from the website until they've worked on a fix.

I imagine the fix has been made for those making future purchases. Those already affected will see their game(s) soon™

Edit: typo

Not everybody was affected. I bought four mystery games and received four. So if there was just a small number of people who didn't get anything, they probably less bad publicity to remove the offer than to keep it.

I also cannot buy any new Mystery Game because I already own it. Looking at the list of Mystery Games I definitly don't ownn every game on that list.

The ones you don't own are likely dispersed in limited quantities throughout the day. At least, this is what happened during their pinata sale. It is really a hit or miss....more misses than not.