bookOutputName variable title

[Gloops]

Hello!
I think it is a Lilypond bug but I let you judge ...
No matter the version of Lilypond from "2.18.2" to "2.21.0"
No matter the version of Frescobaldi from "2.18" to "3.1.1"
On Windows 10 64
Thank you

\version "2.20.0"

title = "test"
\book {
 \bookOutputName \title
 \paper {
 }
 \header {
  title = \title
 }
 \score {
  \relative { c''1 }
  \layout {}
  \midi {}
 }
}

<http://lilypond.1069038.n5.nabble.com/file/t5463/BookOutputName_with_variable.png>
 



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