BUG: An owned game on Steam is making SG mark a different one owned.

That's because you HAVE won it

View attached image.

The above comment fully unravels the mystery.

Not a bug, but the gifter's mistake: you received Pumped BMX Pro instead of the obviously different Pumped BMX+, so now you cannot enter giveaways for the latter because you have previously won it, but Steam Gifts cannot know that you actually received the former (though it does know that you own the former).

Seems there have been an awful lot of such mistakes, not sure if it was marked incorrectly by the bundle site, or -as is often the case with similarly named games- gifters just hastily clicked the first result that appeared in the list... which is, as you can probably guess, Pumped BMX+
(on the bright side, we know for sure that it's a genuine mistake, since Pro is actually considered unbundled

In short, you have a not activated win due to this, so you should provide support with proof of the mistake (partial screenshots of the key in the won page, and of an activation page/prompt for the same key), and get them to mark the giveaway as not received for you (since feedback gets locked after 1 month).

Then, you can sort it out with the gifter, your options here are:

1. they give you a key for Pumped BMX+, you mark the giveaway as received again (or close the ticket if still unanswered);
2. you give them consent to delete the giveaway, they send a delete giveaway ticket (not you, this has to be done by them),
   • "change giveaway game" is not possible, as that option is for different versions (complete, deluxe, GOTY...) of the same game.