And you are sure you are invoking the src/emacs binary produced by
that build?

I'm sorry to ask these obvious questions: it's just that I cannot
understand how the fix could possibly NOT work for you.

The version should be 27.1.50, not 27.1. Could it be that you are
running the original 27.1 binary, not the one built in the src/
directory of the snapshot you unpacked? Can you describe the steps
you used to build Emacs after downloading the snapshot tarball?

Please, don't be sorry, I am new to this so I'm probably doing something wrong...

I've uninstalled my current emacs with pacman -Rns

Then I downloaded the tarball, unpacked it, used the PKGBUILD from emacs version 27.1, I ran makepkg, and then I've used "pacman -U emacs-27..." to install it.