In that case, it's what ASAC_Nobody mentioned. The last giveaway was probably already marked successful (automatic positive review) before you posted here.
So you go to your Profile, then click on Giveaways > Giveaways Completed > Won. You probably already know that, but just in case.
Under the one you won, there should be the name of the giveaway and person who created it etc. You'll see a blank bar and a thumbs up/thumbs down symbol next to it...
The description just says "Rez Plz Steam DEMO key". At least they're honest, I guess?
And I think there was a bunch of giveaways for this game recently too.
https://www.indiegala.com/giveaways/detail/589363
Update: I'm glad to report that the mails/keys that I didn't receive from yesterday's challenges are coming in today.
So far so good, and thank you Support for listening and solving the issue :)
Well I completed 2 challenges successfully today and got that little "Astonishing, you won a key, go to your profile etc" message, but I never received any e-mail or keys on my profile. Has anyone else had this problem?
It was the weekend, so I assume they were unavailable. So they did just answer my e-mail and replaced my invalid keys with new ones.
So just give it some time fellas, Support will eventually take care of everything.