I just upgraded from Sierra to Mojave (skipping High Sierra), so I'm not able to pin down exactly what change caused this, but selecting (by hitting return) text items no longer inserts them at the cursor of the active application.
I used to be able to activate LB, type the name of an emoji, and hit return to insert an emoji. This now appears to do nothing.
I used to be able to activate the clipboard history, select an item and hit return to paste that item. This now appears to do nothing, although if I hit Cmd-V the clipboard history item I selected is pasted.
As far as I can tell, I haven't changed any LB settings, although I dread to think what settings the Mojave ugprade changed.
Does anyone have any suggestions?