A ticket about this won't be needed, as the staff is aware of this, but there's nothing they can do about it; it's a well-known bug of the Steam Web API, which more often than not doesn't correctly recognize DLC ownership.

Since Steam Gifts has to rely on that API, it'll let users enter giveaways for items that aren't shown as part of their libraries (which is the case for most DLCs), as well as packages where they own everything (if they include DLCs).

Now, while most users will only enter giveaways for stuff they don't own, some prefer to use bots and/or scripts to enter giveaways for them, so they won't know which giveaways they joined, nor if they already own what they're trying to win.

With packages you can at least check if the winner owns the base game, and you're allowed to ask for a reroll if they do.
However, with DLCs you'll have to be "creative" - for example, you can temporarily add the winner as a friend and check the store page for "friends who own this"; I don't know what does Valve use for that, but that seems to be working properly.