Sellsword:
I’ve noticed some unusual/inconsistent behavior with ERW too - I think it may have to do with selecting it in the Frost tree, but not selecting it in the DK tree. I had unscpecced it in the main tree to get Abomination Limb back, and then specced into it in the Frost tree, but then the power was greyed out on my bars, and not on either tab of my spellbook.
What fixed it for me was relogging, and then dragging the ERW spell from the Death Knight talent tree (despite that not being where I spent the point) onto my bars.
I think it has to do with the game considering it a Death Knight power first, and not part of the Frost Spellbook tab.
I know that wasn’t an issue a few months ago on beta, but that doesn’t mean thats not the problem now. My first build was as you described, but everything was functional then. Lots can happen, though.