Same here - I have a Philomath Cover sitting in my Vault with the Lucent Night Cover Ornament equipped on it but I am unable to see let alone equip the Ornament on any of my other Warlock helmets!
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This seams to be inline with my experience I had a 1260 Heiros helm with it on but had to dismantle because of .... and once i did thats when i first noticed that i couldn’t refit to other helms.