I found out why it's happening. In all of Cryptic's wisdom, if you ever assign F12 to any keybinding, then remove it later, it will completely remove it from ever being able to rearrange the UI. And there's no way to reassign it. F12 works with an alt that I've never used F12 before. And sure enough, when assigning F12, then removing it, it wipes my ability to ever use it again for rearranging the UI.
Cryptic, can you please fix this ASAP? I'm not going to go into the menu, choose rearrange, then hit Escape over and over again until I get my UI where I want. It's a total pain. Thanks.