Description list items: selective misbehaviour with Ligatures=TeX, fonts, xeLaTeX, and luaLaTeX

I’ve just come across this curiosity. Can anyone tell me what’s going on? Here’s the mwe:

documentclass[12pt]{article}
usepackage{fontspec}
setmonofont{Consolas}
begin{document}
texttt{
begin{description}
item[--help] Help
item[-{}-help] Help
end{description}}
end{document}

And here are the results:

LuaLaTeX:

XeLaTeX:

I get similar results with Source Code Pro in place of Consolas.

Now, I have two issues with this:

1. -()- is, I believe, the canonical way to ‘break’ an automatic ligature. Why is LuaLaTeX producing incorrect output?
2. In defiance of texttt{}, the description list [item]s are being set in Latin Modern Roman, rather than Consolas. Why?

(BTW, I appreciate I may be able to fix #2 with the enumitem package, but I shouldn’t have to, IMO.)

documentclass[12pt]{article}
usepackage{fontspec}
usepackage{enumitem}

setmonofont{Consolas}

newlist{ttdescriptioninner}{description}{1}
setlist[ttdescriptioninner]{font=ttfamily}
newenvironment{ttdescription}
 {ttfamilyttdescriptioninner}
 {endttdescriptioninner}

begin{document}

begin{ttdescription}
item[--help] Help
item[--help] Help
end{ttdescription}

end{document}