ご報告ありがとうございます。
下記のような otf.sty の修整でいかがでしょうか。
ただし、前半は本件とは別件 (https://github.com/t-tk/japanese-otf-uptex/issues/8#issuecomment-1699265979) です。
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diff --git a/japanese-otf-uptex/sty/otf.sty b/japanese-otf-uptex/sty/otf.sty
index 635dedb..f8776bd 100644
--- a/japanese-otf-uptex/sty/otf.sty
+++ b/japanese-otf-uptex/sty/otf.sty
@@ -287,6 +287,7 @@
\newcommand\set@character@{\afterassignment\put@character@\@tempcnta}
\newcommand\put@character@{%
\otf@kanjishape{-}\selectfont%
+ \advance\@tempcnta by "110000
\kchar\@tempcnta%
}
\catcode`\"=\count@
@@ -470,6 +471,8 @@
\DeclareFontShape{\otf@JTn}{hgt}{eb}{n}{<-> s * [\utf@sc@le] \otf@pfx@ \brsg@pfx@ expgotheb\nlck@sfx@-v}{}
\DeclareFontShape{\otf@JYn}{ruby}{l}{n}{<-> s * [\utf@sc@le] \otf@pfx@ \ruby@pfx@ minl-h}{}
\DeclareFontShape{\otf@JTn}{ruby}{l}{n}{<-> s * [\utf@sc@le] \otf@pfx@ \ruby@pfx@ minl-v}{}
+ \DeclareFontShape{\otf@JYn}{rubyg}{eb}{n}{<-> s * [\utf@sc@le] \otf@pfx@ \ruby@pfx@ gotheb-h}{}
+ \DeclareFontShape{\otf@JTn}{rubyg}{eb}{n}{<-> s * [\utf@sc@le] \otf@pfx@ \ruby@pfx@ gotheb-v}{}
\else
\DeclareFontShape{\otf@JYn}{mg}{m}{n}{<-> s * [\utf@sc@le] \otf@pfx@ \brsg@pfx@ nmlmgothr\nlck@sfx@-h}{}
\DeclareFontShape{\otf@JTn}{mg}{m}{n}{<-> s * [\utf@sc@le] \otf@pfx@ \brsg@pfx@ nmlmgothr\nlck@sfx@-v}{}