HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

It's not entirely clear how the developers originally intended this to work. The property itself was likely not added by accident and is probably not a bug (just like the fact that psychic elementals deal half damage to creatures immune to mind magic is not a bug), but it's possible that it was mistakenly assigned to the wrong artifact or was forgotten when artifacts were assigned prices and levels – it's hard to say for sure.

But it's obvious that in the editor and manuals, it's only assigned the property "increases morale by +1".
The artifact is located in a row (in the editor) among the treasure class, and it follows the order: three artifacts for luck, three artifacts for morale.
The artifact can be given at the start (only treasure artifacts are given).
The prices for the artifact on the markets correspond to the treasure price. Therefore, there is evidence that this is a treasure artifact. With properties similar to other artifacts from this class, but in a single direction: either protection from forgetfulness, or from hypnosis, or from berserk. If it's protection from some kind of magic, then it's protection from one type.
But the image of the artifact itself in battle points to a bug. If the pendant against berserk simply prevents it from being cast with a corresponding message in battle, then when berserk is cast on a hero with the courage icon, there is no prohibition, it can be cast, mana is spent, but there is no effect. At the same time, when you cast it on the supposed unit, the blue outline around it is not displayed as it should after other casts. That is, there are a lot of buggy manifestations. In my opinion, they simply mistakenly copied the entire code for protection, which was somewhere nearby.
It is incorrect to draw an analogy with psychic elementals. With elementals, there are a lot of features with vulnerabilities, immunity, which, nevertheless, fit into the model that mind magic is less effective against mindless creatures, undead, or similar ones.
For example, I am more interested in the fact that rings of life with the same property belong to different classes.