(TL;DR readers can read just the second paragraph.)

The developer/website version of the Steam API doesn't send out ownership information on DLCs (as that API can only detect "public" information, which DLCs aren't flagged as), meaning any content that contains at least one DLC won't properly register ownership information. Since SG isn't receiving ownership information on Untold Stories, it interprets the entire package as being unowned (as the only other option would be to block out entry for anyone that owns even a single base game in a package, which is understandably too problematic an alternative).

In short, you'll only ever see DLCs or DLC-including packages register as owned here on SG if you actually won them through SG, as in that case SG can recognize ownership through checking against the win list. (And even then, it'll still show up on the giveaway list, so you do still have to mark them as hidden).

There's also a user-end version of the API, which is the one you use when signing in to Steam Community, which does properly detect DLC ownership. However, that'd require you to remain constantly signed in to that version of the API (a major issue for many of us, given that it's infamous for signing many of us out of community within a matter of hours or even minutes) and would perhaps also increase site demands by way of additional database calls. Those two shortcomings (perhaps along with additional privacy concerns?) are likely why SG doesn't require the user-end API except for initial sign-up (as, for example, SGTools does, due to its need for more in-depth checks on ownership elements).

With that structure in mind, it is possible to use local userscripts to detect DLC ownership for you. EGSGT is the swiss army knife of SG userscripts, so it could be a good one to check if you can't find a more targetted script for the function. I know the more lightweight SG++ script had that function as well, but I don't believe that script is supported or fully functional anymore.

Would also like to know and thank you