You can normally take any general affix from any helm or item and slam it onto class specific. It gives you a message warning it might result in un-usable but it no longer lets you even add the other item to the slot it just makes a noise and puts it back into inventory. Others in game having the same issue. Multiple users = P1!
I wasn’t able to locate any in-game bug reports for this. Could you please submit an IGBR? The in-game bug reporting system automatically enters issues into our triage system, and provides logs and other critical information for debugging issues.