「!」の前の xkanjiskip
ltj-kinsoku.tex には \ltjsetparameter{jaxspmode={\`?,postonly}} がありましたが、 \ltjsetparameter{jaxspmode={`!,postonly}} がありません。
The sequence \ltjsetparameter{jaxspmode={\`?,postonly}} was written in ltj-kinsoku.tex but \ltjsetparameter{jaxspmode={`!,postonly}} was not. Maybe you just forgot it.
Thanks for the report.
The same is true for kinsoku.tex in Japanese pTeX and pLaTeX, so I created an issue https://github.com/texjporg/ptex-base/issues/8.
ltj-kinsoku.tex には \ltjsetparameter{jaxspmode={\`?,postonly}} がありましたが、 \ltjsetparameter{jaxspmode={`!,postonly}} がありません。
The sequence \ltjsetparameter{jaxspmode={\`?,postonly}} was written in ltj-kinsoku.tex but \ltjsetparameter{jaxspmode={`!,postonly}} was not. Maybe you just forgot it.