Hello , i was using Chains of the chosen dead heroic version armor as transmog without a problem like months but it is deleted today from my character except hands and cloak , weirdly they still have the transmog as before , it also happened to my other character using Celestial Observers Ensemble as transmog and already sent a bug report about that.
There is currently an issue which can cause appearances to disappear from the appearances collection, as well as from any items it is currently transmogged to. The appearance will re-appear in the Wardrobe after some time, but can disappear again.
I regret we currently do not have a workaround or resolution, we appreciate your patience while our developers are working to resolve this.