Using \uncover inside a TikZ node text

You need another set of braces:

\documentclass{beamer}
\usepackage{tikz}

\begin{document}
    \begin{frame}
        \begin{tikzpicture}[node distance=2cm]
            \node[align=center] (a) {test 1 \\ {\uncover<2>{test 12345}}};  %wrapped \uncover into {}
            \node[align=center,right of=a] (b) {test 2};
            \path[->] (a) edge (b);
        \end{tikzpicture}
    \end{frame}
\end{document}

\documentclass{beamer}
\usepackage{tikz}

\begin{document}
    \begin{frame}
        \begin{tikzpicture}[node distance=2cm]
          \onslide<1>{%
            \node[align=center] (a) {test 1 \\ \phantom{test 12345}};}
          \onslide<2->{%
            \node[align=center] (a) {test 1 \\ test 12345};}
            \node[align=center,right of=a] (b) {test 2};
            \path[->] (a) edge (b);
        \end{tikzpicture}
    \end{frame}
\end{document}