The $domain_data displays the domain, as expected.
But the $local_part_data is empty.
If I define local_parts as above, then it stays empty.
If I use this definition:
local_parts = leon
it works.
The file is readable and not empty. I can find the $local_part in there.
Even with this command:
su Debian-exim -p -c 'cat /etc/mail/domains/fairtragen.de'
I am using Exim version 4.94.2 #2 built 13-Jul-2021 16:04:57
on a Debian bullseye.