It's simple really; where HUD used to present a list to choose from after the first few keystrokes, it shows no list on 18.04 Unity. I've upgraded from 16.04.
For example, launch Firefox and press ALT. If you type "add" you should see four options listed to choose from on 16.04. On 18.04, I can type "add" into the HUD, but no list appears. Same for anything else I type.
This affects all applications; Libre Office, Nautilus, Gimp etc.
Edit: Looks like my upgrade took me from 16.04 to 17.10 for some reason. The instructions I followed online said it would take me to the next LTS. I'll do another upgrade and see if that improves things.