You mean in the giveaway creation page? Yeah, that's thanks to the Platinum Edition not being actively listed on the Steam store (that is, rather than simply being listed within Steam's internal license database). Since SG works by syncing to store pages, it can only pull information from what's on store pages and even then, only the information that Steam's public API actually allows to be sent out.

<Bundles> aren't configured to send out listing information, nor will any removed games. Likewise anything containing at least one DLC, anything with Profile Features Limited, or anything released within the past 24 hours [assuming PFL hasn't overwritten this element entirely] won't show ownership information correctly, though they will at least send out the rest of their information correctly.

Technically content can be manually added by staff to the giveaway list, but since that ovewrites any syncing elements (and thereby creates a lot more work for staff if future syncing is necessary), staff only ever does that for content which they're certain won't ever be added/readded to Steam [like games removed from Steam due to the associated developers or publishers being banned].

And then you've got the developer/publisher misconfigurations, and the fact that the Steam store is a clunkily coded mess to begin with, and has several weird quirks that affect information displayed in the store (an effect that is then transferred to any API calls on the affected pages- notably, if you ever see a package on a page go on sale while the base game on that page doesn't, and the price of the package ends up being cheaper than the base game, you'll then notice that any store searches for the base game will instead return the information for the given package).

On a final note, SG rules state that you need to provide whatever you list under, but that you can provide anything on top of that as you desire. So there's no issue at all posting the Platinum Edition as the base game. However, if you request the winner to leave a comment as to what version they received, then if the Platinium Edition is ever added to Steam/SG you can just create a "Change Giveaway Game" ticket linking to that comment to get the game changed to the (presumably) higher value Platinum Edition.

Hopefully that all wasn't too much to throw out at you, and hope at least some of it was helpful or at least informative. :)
Appreciate you putting this game up for us, and have a warm welcome to Steamgifts! ^.^

As a final note, if you're aiming to win some games, forum and group giveaways generally have better odds than low level public ones. I'd turn special attention to Unlucky 7, as that caters very well to new users (who have either been on the site for two months or given away seven games of their own).

View attached image.