>
> 1. Evaluate: (setopt completions-header-format nil completion-show-help 
> nil)
>
> 2. Type M-x followed by TAB. The *Completions* buffer pops up and shows 
> all available commands.
>
> 3. Type M-<down>.
>
> Expected result: The first candidate in *Completions* is selected.
>
> Actual result: The second candidate in *Completions* is selected.
>

Thanks for your bug report.

I attach a patch to fix that bug, can you please try it?

It's a workaround, but it's the cleanest fix I can think of.