I had it unlocked ever since Season of the Seraph and recently when I was changing up my guardian's look to more of a mech-type aesthetic I wanted to use it but couldn't find and it turns out it is straight up missing.
It might not be a known issue but I am sure it's not just me having this problem and I'd really like it back.
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Hey there. Worn items do not show in the transmog list. Your Hunter is wearing an actual Warmind's Avatar Vest (which may well be ornamented as something else), which is the reason you do not see it.