I noticed that at least two cloaks were not displayed by ATT as "missing" on my chars below the Events-->13th Anniverary-->World bosses.
I did not have them and got them from daily bosses this week for the 15th anniversary (Nov.2019).

Drape of Benediction (Azuregos)
Green Dragonskin Cloak (Emerald dragon)

There are at least two more cloaks I miss, Puissant cape (Azuregos) and Eskhandar's Pelt (Kazzak) and they also not show in ATT

I've got the filter "Cloak" enabled in the Filters tab.

Think it's a bug, or is it something that was considered only for the 13th anniversary, but it's still obtainable atm in 2019, during the 15th anniv.

I have all unique mode boxes disabled.
The ones I still miss are 150380 and the other one is only in wowhead classic loot tables, so looks like the reason why not showing up in ATT (not even in Debug mode).

For 150380 If I enable Completionist mode , it shows up, says I've collected it but not from this item. So it's as you say, a shared appearance.

If you have a Shared Appearance and are playing on Unique Mode, that's working as intended. Please show the tooltip for the two cloaks you are missing.

Puissant Cape (18541) is not obtainable since the original Azuregos was removed.
Eskhander's Pelt has two versions now (18204 - unobtainable from original Kazzak) and (150380 - from anniversary Kazzak).
Drape of Benediction (150417) is obtainable from anniversary Azuregos and is in ATT
Green Dragonskin Cloak (150411) is obtainable from anniversary green dragons, and is in ATT.

All of these are accurately listed in ATT at this time, so I'm not sure there's any need for this issue still.