Looking closer, this seems a mode-specific issue: in harf mode, "instance=Light Extended" works just fine, but in (default) node mode it doesn't.
Attached is the output of
\input luaotfload.sty
\parindent=0pt
\parskip=0pt
\def\fonttest#1{%
\font\testfont="#1"\relax
{\testfont ABCDEFGHIJKLM}\par}
\fonttest{Skia}
\fonttest{Skia:instance=Light Extended}
\fonttest{Skia:mode=harf;instance=Light Extended}
\bigskip
\fonttest{[STIXTwoText-Italic.ttf]}
\fonttest{[STIXTwoText-Italic.ttf]:instance=SemiBold Italic}
\fonttest{[STIXTwoText-Italic.ttf]:mode=harf;instance=SemiBold Italic}
\bye
Skia.ttf is Apple's Skia font, and STIXTwoText-Italic.ttf is
STIXTwoText-Italic[wght].ttf from the STIX Two fonts release 2.13b171, renamed.
Bruno Voisin
instance-test.pdf
Description: Adobe PDF document
instance-test.tex
Description: Binary data
