Compelled Duel Bug

[Memento Moriarty]

I compelled a monster with Wyll who I turned into a Paladin/Warlock. The monster just shrugged and booted Shadowheart OFF A CLIFF TO HER DEATH.

What gives? I checked the monster's status afterward, and it still had compelled duel on it.

[jsaving]

The flavor text implies monsters can't attack anyone but you, which would be incredibly powerful in many circumstances. However the actual impact of the spell is to impose disadvantage on those attacks. That isn't nothing but it generally won't be enough to protect vulnerable party members if the AI sees a tactical opportunity.

You'd typically be better-served using that spell slot to power a smite that could potentially make your foe prone or frightened. (Or alternatively you could go with sanctuary if you picked a paladin order capable of casting it.)