\Hy@DisableOption in \PDF@FinishDoc is local, hence do nothing

Currently, hyperref, at the time of first page break, uses \PDF@FinishDoc to set PDF info fields like pdftitle and make related options disable. After that, any key-val setting to options like pdftitle will not influence the corresponding field in the output PDF, and should raise warning \Hy@WarnOptionDisabled.

But, since

• the \PDF@FinishDoc is executed inside commands of atbegshi package, and
• the \define@key (provided by keyval package) used by \Hy@DisableOption only locally (re)defines key,

options like pdftitle are never disabled, hence usages after first page break won't raise any warning.

Following is a quick fix (with test by showing the meaning of \KV@Hyp@pdftitle) by providing global variant of \define@key and using it inside \PDF@FinishDoc.

• Without fix, \KV@Hyp@pdftitle is not redefined after the first page break.
• With the fix, \KV@Hyp@pdftitle is redefined as desired and warning \Hy@WarnOptionDisabled is raised after the first page break.

\documentclass{article}
\usepackage{hyperref}
\hypersetup{pdftitle={preamble}}

\makeatletter
%% redifined to use \define@key@g
\def\Hy@DisableOption#1{%
  \ltx@ifundefined{KV@Hyp@#1@default}{%
    \define@key@g{Hyp}{#1}%
  }{%
    \define@key@g{Hyp}{#1}[]%
  }%
  {\Hy@WarnOptionDisabled{#1}}%
}

%% add \global version of \defin@key
\def\define@key@g#1#2{%
  \@ifnextchar[{\KV@def@g{#1}{#2}}{\global\long\@namedef{KV@#1@#2}####1}}

%% add \global version of \KV@def
\def\KV@def@g#1#2[#3]{%
  \global\long\@namedef{KV@#1@#2@default\expandafter}\expandafter
  {\csname KV@#1@#2\endcsname{#3}}%
  \global\long\@namedef{KV@#1@#2}##1}
\makeatother
\listfiles


\begin{document}
%% this one succeeds in changing the Title field
\hypersetup{pdftitle={before first newpage}}
a
\newpage

%% with fix, this one raises warning
\hypersetup{pdftitle={after first newpage}}

%% check meaing of \KV@Hyp@pdftitle
\makeatletter
\ttfamily
\meaning\KV@Hyp@pdftitle
\makeatother
\end{document}

This issue is first reported, in Chinese, in https://github.com/CTeX-org/forum/issues/42.

Trace \PDF@FinishDoc, \Hy@DisableOption, and the key definitions from keyval and atbegshi to identify where local scoping prevents the option handler from being replaced. Use the minimal document in the issue to compare \KV@Hyp@pdftitle before and after the first page break; done means the handler is disabled globally and \Hy@WarnOptionDisabled is raised afterward.