Zsh _normal completion inconsistent

Question

I've got a completion function which at some point is supposed to revert to completing another command. Think sudo another_command style.

  ...
      case $subcommand in
        (exec)
          _arguments -S -C \
            '(-p --profile)'{-p,--profile}'[something]: :' \
            '*::cmd:->cmd' && ret=0
          case $state in
            (cmd)
              _normal -P
              ;;
          esac
  ...

This behaves as follows:

• command exec <tab> - correct, completes new command
• command exec -p foo <tab> - completes new command
• command exec -p foo -- <tab> - correct, completes new command
• command exec -- <tab> - wrong, suggests local files

I'm confused what's different about the last case - I expected -S to ignore -- like in the previous case. Why doesn't it? (the code does get to the _normal -P line correctly in all cases, echo-debugging confirmed it)

_complete_help at the wrong completion shows:

tags in context :completion::complete:--::
    globbed-files  (_files _default)

and at the right completion:

tags in context :completion::complete:-command-::
    commands builtins functions aliases suffix-aliases reserved-words jobs parameters parameters  (_command_names _autocd) 
    commands                                                                                      (_path_commands _command_names _autocd) 
    jobs                                                                                          (_jobs _command_names _autocd) 
    local-directories