Only recently have I noticed this issue. I can't custom keybind anything with a + action, such as Alt + 5. It shows either Alt or 5, but not combined. I have keybinds that work fine already with Alt+1, Shift+K, etc. I uninstalled some recent addons, but no change.
I asked in some guilds and someone suggested my Graphics drivers were bad or my game needed to be repaired. SO I reinstalled the drivers and repaired the game...and it works...on select characters, but not all characters.
The inability to keybind with two keys only occurs if the addon alphagear is disabled. Once enabled I can keybind again. Oddly it must have some sort of hold even when you turn addons off.