Given a trivial completion function (as a simple reproduce case):

function foo() {
        echo $COMP_WORDS
}
complete -F foo foo

Try typing: foo a{1,2} bar<tab>

Expected is that after pressing tab, it outputs "bar".

Instead, it will find_cmd_start() at the offset of "1,2}", not find the assigned completion and attempt default completions instead.

The issue is that { is here recognized as a command separator, by itself, while it only really is a command separator if at start of line (excluding whitespace) and after other command separators.
This needs special handling.