echo "$((cmd "$@") 2>&1)"

echo "$( (cmd "$@") 2>&1)"

You appear to be using $(( with two (or more) parentheses in a row, where the first $( should open a subshell.

This is an ill-defined structure that is parsed differently between different shells and shell versions. Prefer adding spaces to make it unambiguous, both to shells and humans.

Consider the $((( in $(((1)) ):

Ash, dash and Bash 1 parses it as $(( ( and subsequently fail to find the matching )). Zsh and Bash 2+ looks ahead and parses it as $( ((. Ksh parses it as $( ( (.

Alternatively, you may indeed have correctly spaced your parentheses, but ShellCheck failed to parse $(( as an arithmetic expression while accidentally succeeding in parsing it as $( + (.

In these cases, double check the syntax to ensure ShellCheck can parse the $((, or ignore this error and hope that it won't affect analysis too severely.