Have you ever had butter mold?

Butter does not get moldy. It will get rancid if not properly stored. Possibly some food source (ex. bread crumbs) was left on the butter and that is what turned to mold.

Rancid: from dictionary.com;
(of foods containing fat or oil) smelling or tasting unpleasant as a result of being old and stale.

I’ve had a Jello mold. I didn’t much care for it.

I’ve had mold on butter, and @AmWiser, there was no other food source on it. It was on butter that we use exclusively for cooking and a clean knife is always used to cut it.

Another possibility is that the OP’s “premium” butter was not pure butter. Nowadays they make butter with all sorts of other ingredients in it.