TikZ / calendar: Problem mit multirow in node

\documentclass[landscape]{scrartcl}
\usepackage[margin=1.75cm,showframe=false]{geometry}
\pagestyle{empty}

\usepackage{tikz}
\usepackage{array, multirow}
\usetikzlibrary{calendar}

\pgfkeys{tikz/.cd,
         my inner sep/.store in =\myinnersep,
         my inner sep=5pt
}

\def\CalendarNumbersFont{\Huge\sffamily}%\bfseries
\begin{document}
SOLL:  \begin{tabular}{l l l }
\multirow{1}{*}{\Huge 2} & a  & \multirow{1}{*}{\Huge M}\\
{} & b & {}
\end{tabular} \\[10cm]
IST:

% Schrifthöhe auslesen
\CalendarNumbersFont \pgfmathsetmacro{\myheight}{height("1")}
\typeout{\myheight}

\newcommand\Tabelle{\begin{tabular}{l >{\footnotesize} l l}
\multirow{1}{*}{\tikzdaytext} & a  & \multirow{1}{*}{\Huge M}\\
{} & b & {}
\end{tabular}}

\centering
\begin{tikzpicture}[
  every day/.append style={
    font=\CalendarNumbersFont,
    draw,
    inner sep=\myinnersep,
    text width=(\textwidth-\pgflinewidth)/7-2*\myinnersep,%
    text depth=(\textheight-\pgflinewidth)/5-\myheight-2*\myinnersep,%
    minimum width=(\textwidth-\pgflinewidth)/7 ,%
    minimum height=(\textheight-\pgflinewidth)/5,%
},
]
\calendar (cal)
[
  day xshift=(\textwidth-\pgflinewidth)/7,% <- eingefügt
  day yshift=(\textheight-\pgflinewidth)/5,% <- eingefügt
  dates=2018-08-01 to 2018-08-last,
  week list, 
]
%if (Sunday) [nodes={red, fill=pink}]
if (equals=2018-08-02) [
nodes={red, fill=pink}, 
%day code={\node[every day]{ \Tabelle  };}
day code={\node[every day]{\tikzdaytext$\vphantom{\tikzdaytext}_{{\Huge b}}^{a}$ };}
]
%
if (equals=2018-08-07) [
nodes={red, fill=pink}, 
day code={\node[every day]{ \Tabelle  };}
]
;

\fill[red, thick] (cal-2018-08-20) circle (7pt);
\node[red, draw, align =left] (Termin) at (cal-2018-08-21) {Termin\\ Termin};

\node[] {\begingroup
\begin{tabular}{l >{\footnotesize}l l }
\multirow{1}{*}{\Huge M} & aaaa  & \multirow{2}{*}{ M}\\
{} & bbb & {}
\end{tabular}
\endgroup
};
\end{tikzpicture}

\end{document}

\documentclass[border=5pt, varwidth]{standalone}
\usepackage{multirow}
\usepackage{array}

\usepackage{amsmath, tikz}

\begin{document}
%schlecht: M$\vphantom{M}^a_b$

\newcommand\Tabelle{
\begin{tabular}{l >{\footnotesize}l l }
\multirow{1}{*}{\Huge M} & aaaa  & \multirow{2}{*}{ M}\\
{} & bbb & {}
\end{tabular}
}
\Tabelle

\tikz[]{\node{\Tabelle};}

\end{document}