Sadly the better fix did not work

so go back to mokeypatching

manual/latex.py

@@ -10,17 +10,29 @@ import os
 
 from sphinx.builders.latex import LaTeXBuilder
 from sphinx.util.logging import getLogger
+from sphinx.writers.latex import LaTeXTranslator
 
 
 def info(*a):
     getLogger(__name__).info(*a)
 
 
+class FixedLaTeXTranslator(LaTeXTranslator):
+    # see https://github.com/sphinx-doc/sphinx/issues/8936
+
+    def visit_substitution_definition(self, node):
+        pass
+
+    def depart_substitution_definition(self, node):
+        pass
+
+
 class LaTeXHelpBuilder(LaTeXBuilder):
     name = 'mylatex'
+    default_translator_class = FixedLaTeXTranslator
 
     def finish(self):
-        LaTeXBuilder.finish(self)
+        super().finish()
         info('Fixing Cyrillic characters...')
         tex = os.path.join(self.outdir, 'calibre.tex')
         with open(tex, 'r+b') as f: