Unable to cast Arc when fitted to Mjolnir

I have a Mjolnir and when I load an Arc gem - If I map the Arc gem to a button then when I mouse over it I get the message [Skill can't be used].

I have tried with a Corrupted Vaal gem - where the Vaal Arc skill is available but Arc is not

I have tried with a brand new standard gem bought from a Vendor - [Skill can't be used].

I have loaded the gems into another weapon where they work as expected

This is not a bug.

Active skills supported by triggers cannot be cast manually. As Mjölner states "Trigger a Socketed Lightning Spell on Hit", socketed Lightning spells placed within it are supported by that trigger. Thus, the ability to cast them manually is disabled.