It is a known issue. I had the helms disappear on me last night – they are back now. Also had the Renowned Explorer mog disappear too. Other people have reported the Sprite Darter mog from the Shop disappearing as well.
Recent post from Blizzard: Spirit Darter transmog removed? - #7 by Kalviery
Support articles about missing mogs:
Cosmetic Helms - https://us.battle.net/support/en/article/17744
Sprite Darter - https://us.battle.net/support/en/article/291576
Renowned Explorer - https://us.battle.net/support/en/article/264950