That is because you have a Diamond in your helm.
Please read: Things That Are NOT Bugs Stickied at the top of the Bug Report Forums
Putting the Crown on your follow is the same as putting it in the cube. When it is in the Cube or on a follower, YOU get the power. Therefore the Gem in YOUR helm determines the gem type that is dropped, just like it says in the description.
Whenever a gem drops, a gem of the type socketed into your helmet also drops. This effect does not apply to Legendary Gems. (Emanates)
It does not say the type of gem socketed in the follower’s helmet will drop, nor does it say the type of gem socketed into the Broken Crown will drop. It says “your helmet” for a reason.
This is not a bug.