Right, but that's a Steam API issue- many DLCs on Steam aren't properly tagged in their settings in what base games they require. In other words, blame Valve's sloppy design or the publisher's lazy implementation of the product details.

As an example of that, as I recall from previous SG discussions on the topic, region-localized DLCs are sometimes tagged as belonging only to the base game of that region. Even if they're compatible with the base game purchased in another region, they won't necessarily send out the information in the API for their associated base game except for the version specific to that region.
(Put another way, Steam reports the DLC as only being linked to a specific base game, and you don't have that specific version. SG doesn't know to check for any other versions because of that.)

Another example is DLCs to DLCs- since the API doesn't properly send out DLC ownership information, and the DLC requires an existing DLC, there's no way for SG to detect ownership correctly. Meanwhile, Standalone Expansions often send out a weird mix of information in regards to if it's a DLC or a base game, and that can cause a lot of issues as well.

In any case, there's absolutely nothing SG can do about such things [other than occasionally manually link the DLCs, which does require you to send in support tickets so they know to do such] so there's no point in blaming them for what's just another poor Valve design.

tl;dr It's not a bug, it's poor design (on Steam's end)