I just finished the Arathi Warfront and I got the 7th Legionnaire's Greatbelt, the item ID is 163422. On the attached screen you can see that this item I am wearing and it is Soulbound, when I hover mouse of this item, it displays tool tip (the left one) that the item 163422 is learned - see blue texts. But in ALL THE Things it is still displayed that it is not yet learned and you can see on the right tooltip that item ID 163422 is not yet leared.
I tried Shift Click on that item to refresh database, but without success.

Game master explaned me that the item received from Victory is not eligable for Transmog sets, so the tool tops are correct, one displays possible drops eligable item and second tooltip displays all possible drops (eligable and non-eligable items).

Do you have 1.7.7 released the other day? I think it was fixed in that release

Yes, I am using 1.7.7 and during warfront I already had installed and active this version.

I also tried to log this problem on Blizzard support page and their AI told me this:
“Items obtained from the completion of a Warfront do not count toward any set. Sets for Warfronts include items that drop off rares in Arathi, participating in PvP, and caches from Emissaries.”

If I understood the message correctly, the drop from Victory is not used for Appearance set “7th Legionnaire’s Plate”.

It looks like the problem is in tooltip when I open Appearances Set and hover mouse over non-learned item on the Waist item, it suggest 5 different ways to receive this item:
ATT->Expension Features->War Effort->Quests->Death to the Defilters->Warfront Equipment Cache
ATT->Expension Features->War Effort->Common Boss Drop
ATT->Expension Features->War Effort->Warfront: The Battle for Stormgarde->Victory
ATT->Zone->KulTiras->Boralus->War Effort->Quests-> Warfront: The Battle for Stormgarde-> Warfront Equipment Cache
ATT->Zone->KulTiras->Boralus->Emissary->Alliance War Effort

I received this item from the italic/bold line, but it looks look, according to Blizzard AI this way, the items could not be added to the Appearnce set.
It is confusing to me, when I hover mouse over ATT item '7th Leggionaire's Greatbelt', it displays only 3 ways to get it and the Victory is not there.
Name is the same, item ID is the same, but the tooltips are different. Confusing.