I am reading that items that have enchants that cannot be replaced are supposed to show a little lock icon under the enchant bar (between the bar and the enchant name). However, I have an item called Ancient Warden's Greaves that does not show this lock icon. But it won't let me overwrite the "adds 21 max health" enchant that's on it.

I'm not quite understanding how this works. I assume since there is no lock icon that the enchantment can be replaced, but it still won't let me.