usetagform works inside equation but not work inside align

\documentclass{article}\usepackage{amsmath}\usepackage{mathtools}\newtagform{mytag}{\{}{\}}\newcounter{mycn}\newcommand*{\mysym}{\refstepcounter{mycn}\usetagform{mytag}\tag{\themycn}}\begin{document}    \begin{equation}        a=b    \end{equation}    \begin{equation}        c=d \mysym    \end{equation}    \begin{align}        a & b \\        b & c    \end{align}    \begin{align}        a & b \mysym \\        b & c \mysym    \end{align}\end{document}