documentclass{book}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage[english]{babel}
usepackage[]{reledmac}
usepackage{xstring}

%%% A dummy macro
newcommand{mymacroA}[1]{textbf{#1}}

newcommand{morenoexpands}{%
   letmymacroArelax
}%

makeatletter
newcommand{truncatelemma}[1]{%
   begingroup%
   morenoexpands%
   normalexpandarg%
   StrCount{#1}{ }[@numspaces]%
   ifnum@numspaces>5%
      noindentignorespaces%
      StrBefore[3]{#1}{ }%
       ldots{} %
      StrBehind[numexpr@numspaces-2relax]{#1}{ }%
   else%
      noindentignorespaces%
      #1%
   fi%
   endgroup%
}%

Xwraplemma[A]{truncatelemma}

deffootnoteAnmerkung{@ifstar@footnoteAnmerkung@@footnoteAnmerkung}
newcommand{@@footnoteAnmerkung}[3][]{%
   ifcsdef{lemmastart#1}%
      {xdef@tempA{#2}%
          %%% xpreto vs. gpreto
         gpreto{@tempA}{csuse{lemmastart#1} }%
         #2%
         edtext{}{%
            expandafterlemmaexpandafter{@tempA}%
            Afootnote{noindentignorespacescsuse{anmtext#1}}%
         }%
    }%
    {edtext{#2}{%
     Afootnote{noindentignorespaces#3}%
    }%
    }%
}%
newcommand{@footnoteAnmerkung}[3][]{%
   ifstrempty{#1}%
      {% ERROR
      }%
      {%
         #2%
         csgdef{lemmastart#1}{#2}%
         csgdef{anmtext#1}{#3}%
      }%
}%
makeatother

begin{document}
beginnumbering
autopar

This is footnoteAnmerkung*[N1]{almost the same text}{now we have a
split note} with some text in between and footnoteAnmerkung[N1]{to show   the
problem with expansion}{}. That's it.

%% Not working with xpreto above
This is footnoteAnmerkung*[N1]{almost mymacroA{the same} text}{now we have a
split note} with some text in between and footnoteAnmerkung[N1]{to show the
problem with expansion}{}. That's it.

endnumbering

end{document}