Hi,

in EB Garamond I have some contextual lookups in the kern feature with 
which LuaTeX in context standalone (updated today in the morning) seems 
to have problems (the issue is not new, I’ve already reported it in 
january but perhaps that was a bit too confusing).

The test document is:

\definefontfeature[default][default][mode=base]
\definefontsynonym[ebg][file:EBGaramond12-Regular.otf]
\definefontfeature[ss20][ss20=yes]
\definefontfeature[iota1][cv81=1]
\definefontfeature[iota2][cv81=2]
\definefontfeature[altcirc][cv80=1]
\define\test{Tì αᾟα\par}
\starttext
\definedfont[ebg*default at 12pt]\test
\addfeature{ss20}\test
\addfeature{iota1}\test
\addfeature{iota2}\test
\addfeature{altcirc}\test
\stoptext

The font is at 
http://www.georgduffner.at/ebgaramond/fonts/EBGaramond12-Regular.otf

The corresponding lookups in the kern.fea roughly contain

     pos T' 145 i.dotless gravecomb;

which increases the right sidebearing of T by 145 before i.dotless 
followed by gravecomb

and

     pos Eta' <218 0 218 0> uni0345.cap @grk_tlAcc2;
     pos Eta' <218 0 418 0> uni0345.cap1 @grk_tlAcc2;
     pos Eta' <218 0 558 0> uni0345.cap2 @grk_tlAcc2;

where @grk_tlAcc2 contains the accents attached at the top left of the 
glyph and which means that Eta is positioned at 218 inside its bounding 
box and the bounding box is expanded by the third value between <>, on 
the condition that it’s followed by uni0345.cap* and the said accents.

ss20 does a full decomposition of all composed glyphs so I can replace 
any component by an alternate glyph. Their order corresponds to 
canonical decomposition.

Attached you find the pdf output and a pdf from a parallel xelatex test 
which shows the desired result:

\documentclass[12pt]{minimal}
\usepackage{fontspec}
\setmainfont{EB Garamond}
\newcommand{\test}{Tì αᾟα\par}
\begin{document}
\test
\addfontfeature{RawFeature=+ss20}\test
\addfontfeature{CharacterVariant=81:1}\test
\addfontfeature{CharacterVariant=81:2}\test
\addfontfeature{CharacterVariant=80}\test
\end{document}

Best regards,
Georg