Comment (by Roland Haas):

Turns out this was a bug introduced in git hash 561cf871 "Cactus: make quoting of strings more conservative in ThornDoc" of cactus in 2017. This change incorrectly made the ThornDoc script incorrectly end all documentation files with \\end{document} rather than \end{document} and for some reason this only seems to have cause an issue in Formaline’s thorndoc.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2296/thorn-documentation-displayed-on-website