test.tex

\documentclass{article}
\usepackage{expand-control}
\begin{document}
\makeatletter

\errorcontextlines=5
\tracingmacros=1
\edef\ak{\uppercase{abc\PreserveCase{ }def}}\show\ak
\tracingmacros=0\relax
\MakeUppercase{abc\PreserveCase{aABb}def}


\message{\detokenize\expandafter{%
  \expanded{\expand@three}\expanded{\unexpanded{\char@generate{97}{12}}\expand@three}%
  \expanded{\unexpanded{\char@generate{97}{10}}\expand@three}%
  % \expanded{\noexpand\@gobble\expand@three}%
  \expanded{\unexpanded{\char@generate{65}{1}}\expand@three}%
  \expanded{\unexpanded{\char@generate{65}{2}}\expand@three}%
}}

\edef\a{\uppercase{%
  \unexpanded{\expanded{\unexpanded{\char@generate{97}{12}}\expand@three}%
  \expanded{\unexpanded{\char@generate{97}{10}}\expand@three}%
  % \expanded{\noexpand\@gobble\expand@three}%
  \expanded{\unexpanded{\char@generate{65}{1}}\expand@three}%
  \expanded{\unexpanded{\char@generate{65}{2}}\expand@three}%
  }%
}}\show\a

\edef\a{\unexpanded{\char@generate{97}{12}}}\show\a

\end{document}