/etc/fonts/conf.d/31-cantarell.conf is in fonts-cantarell 0.0.15-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/fonts/conf.d/../conf.avail/31-cantarell.conf
/etc/fonts/conf.d/../conf.avail/31-cantarell.conf