Apologies, seems I forgot to reply to all. See my message below. 

Tl:dr: my OS had indeed a locale problem which I now corrected. Now it all works well. Thanks for your help and sorry for the false report.

On Mon, Oct 11, 2021, 15:14 Darío Cuevas <superdarion@gmail.com> wrote:
That was a quick reply.

I checked and my locale was wrong (some values were set to "C",
instead of the de_DE.UTF-8 of the rest), caused by some bad KDE plasma
configuration file. It actually helped me solve an unrelated problem
I've had for a while, so thanks a lot!

Sorry to have reported it as a bug when it was my system's fault.

Best regards,
Darío


El lun, 11 oct 2021 a las 14:14, Roman Perepelitsa
(<roman.perepelitsa@gmail.com>) escribió:
>
> On Mon, 11 Oct 2021 at 14:05, Darío Cuevas <superdarion@gmail.com> wrote:
>>
>> To reproduce, one
>> needs only do PROMPT="→" on .zshrc, open a terminal, start typing some
>> command (e.g. ls) and hit TAB: the command (ls) will be written twice.
>
>
> This usually happens when you put non-ascii characters in PROMPT (like that arrow) but your locale is not UTF-8. Run `locale` to confirm. To fix this, install and enable a UTF-8 locale.
>
> This can also happen when TERM environment variable doesn't match the real terminal type. To fix this you need to avoid overwriting the value of TERM set by the terminal.
>
> Roman.



--
Yes, indeed.