With this addon enabled and searching for sets and extra sets, only the ones already collected shows up in the results.

Uncollected or partially collected sets are not shown in the results.

In the default UI none collected sets shows up on search correctly.

Depends on the sets, some of the older expansion sets show up as partial sets, those the item is displayed above the dressing room model and you have to mouse over the item to see if you actually have it or not.

I do not have the helm for the Overlord's Plate set (a classic era set), but it shows like the pieces I do have.

while the world defiler set from Legion doesn't show in the list in the appearances tab, it does show in the transmog window.

@Grokironhide that is not the same issue. I suggest you to make a new separate issue instead.

I am talking about the search function, searching for sets by name in the search box.