I got incorrect manpages from the code:
..., std::string const &delimiters = std::string (" \t\v\f\a\n\r\0", 8));

Which results in a manpage containing:
std::string(' \\t\\v\\f\\a\\n\\r\\0', 8))

(As an aside, the double quotes have been replaces with single quotes, which is not correct C++)

The problem is that man appearantly evaluates this thing three times before displaying. This means that every backslash must be thrice doubled, leading to 8 backslashes:
' \\\\\\\\t\\\\\\\\v\\\\\\\\f\\\\\\\\a\\\\\\\\n\\\\\\\\r\\\\\\\\0'

This gives the result of a single backslash in the manpage as shown by man and no more warnings from man.

Arguably this is a bug in man. However, given that everybody uses it with the current syntax, I don't think it's realistic to request a parser change for it.