I don't actually know if it happens with other enchantments, but after you dismount, the vorpal enchantment visual effect stand in your hand while you're not in combat.
For you that have pain while watching this, know that if you dismount using the mount button (6 by default) your enchantment will not bug. This is why it bugs on PvP and not on PvE, since you always dismount by pressing a button to attack.
Just for your information
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