Modified align environment is not rendered properly

[leanhdung1994]

I have a tex file

\documentclass[12pt]{article}
\usepackage{amsthm,amssymb,mathtools}

% Create modification of align environments
% https://tex.stackexchange.com/a/751147/161639
% https://tex.stackexchange.com/a/751622/161639
\makeatletter
\ExplSyntaxOn
    %% Variable declarations.
    \int_new:N \g__example_position_int

    %% The list of environments to define.
    \prop_const_from_keyval:Nn \c__example_envs_prop {
        myalign = align,
        myaligned = aligned,
        myalign* = align*,
        myaligned* = aligned*,
        myequation = equation,
    }

    \prop_map_inline:Nn \c__example_envs_prop {
        %% Define the new environments.
        \NewEnvironmentCopy { #1 } { #2 }

        %% Inside the environments, make `&` active and make have it run
        %% the `\__example_align:` command.
        \hook_gput_code:nnn { env / #1 / begin } { . } {
            \zcsetup { currentcounter = equation }
            \int_gzero:N \g__example_position_int
            \char_set_catcode_active:N \&
            \char_set_active_eq:NN \& \__example_align:
        }
    }

    %% Workaround for https://github.com/latex3/latex3/issues/1796
    \cs_new:Nn \__example_only_align: {
        \c_alignment_token
    }

    %% Also make `&` active inside the specified environments, so that nested
    %% `myaligned`s work inside of them.
    \clist_map_inline:nn { align, align*, equation, equation* } {
        \hook_gput_code:nnn { env / #1 / begin } { . } {
            \char_set_catcode_active:N \&
            \char_set_active_eq:NN \& \__example_only_align:
        }
    }

    %% Reset the position counter after measurement is done, otherwise the
    %% counter will be twice as large as it should be.
    \hook_gput_code:nnn { cmd / measure@ / after } { . } {
        \int_gzero:N \g__example_position_int
    }

    %% Define the `\__example_align:` that runs every time `&` is encountered.
    \cs_new_protected:Nn \__example_align: {
        %% Increment the position counter.
        \int_gincr:N \g__example_position_int

        %% Equivalent to a regular alignment `&`.
        \c_alignment_token

        %% If this is not the first `&`, insert a `\quad`.
        \bool_if:nT {
            \prop_if_in_p:NV \c__example_envs_prop \@currenvir &&
            \int_compare_p:n { \g__example_position_int > 1}
        } {
            \quad
        }
    }
\ExplSyntaxOff
\makeatother

\begin{document}

\begin{myalign}
& a \\
& = \begin{myaligned}
& b + c \\
& + d + e .
\end{myaligned} 
\end{myalign}

\end{document}

It seems the user-defined environments myalign and myaligned are not "loaded":

Could you have a check on this issue?