The same "nothing selected" behaviour I saw in the Spellchecker, is in the ToC Editor and the Metadata Editor - as can been seen below:
This
rather than
This
rather than
That suggests to me that it is a behaviour inherited by those dialogues from underlying object classes - presumably from the Qt library.
In the ToC Editor, if I don't hit space to select the "Title" line entry then… when I tab to the Down arrow button and hit space the "Title" line entry doesn't move down… because nothing is selected.
Added: Given that's how it's always been I DO NOT regard this as a bug.
Other dialogues containing lists are the same… e.g Reports, Keyboard shortcuts.
I suspect I misread the discussion about the ToC Editor buttons.
BR