I've noticed that you can get Thorns enchant on boots (and leggings) by enchanting but if you take them off you can't put them back on from GUI. I'm not sure if this is intended functionality or a bug.

some enchants are like that you just have to apply them again using an Anvil

and seriously Thorns ? why do you even need any enchantment beside EIO soul-Bound ?

Besides the point. I just happened to notice that this occurs while I was playing around with enchants and wondered if it's a bug.

Not sure how you are able to put it on your boots with an enchanting table but thorns is a chestplace only enchant which is why you cant reapply it via the gui.