Given the following:
\lecture{Shortname}{Longname}{Label}
\begin(frame}
\titlepage
\end{frame}
How does beamer infer to insert the first argument of \lecture to be the title used in \titlepage?
I've searched my texlive installation (texlive-beamer-svn36461.3.36-24.fc24.1.noarch) but couldn't find where or how \beamer@shortlecturename is turned into \inserttitle?
What I would like to accomplish is that Longname is used in \titlpage instead of Shortname.
Update:
Turns out \lecture is not the culprit here. An MWE of what I'm actually doing looks like this:
\documentclass{beamer}
\newcommand{\titlestring}{Title}
\title{\titlestring}
\begin{document}
\begin{frame}
\titlepage
\end{frame}
\renewcommand{\titlestring}{Topic}
\lecture[\titlestring]{\titlestring, YYYY-MM-DD}{Label}
\begin{frame}
\titlepage
\end{frame}
\end{document}
What I still don't get is why \titlpage picks up the changed \titlestring.

\lecture[<short>]{<long>}{<label>}- but the defaultbeamertheme doesn't include<shortname>as part of the\titlepage. – Werner Oct 14 '16 at 20:48