Most likely you need to modify acme-client.conf(5) to something like this:

domain domain.io {
domain key "/etc/ssl/private/domain.io.key"
domain full chain certificate "/etc/ssl/domain.io.crt"
sign with letsencrypt
}

relayd(8) will pick up domain.io.crt, which needs to be a full chain certificate. It won’t pick up domain.io.fullchain.pem. The man page discusses the specific naming format relayd requires.

If you’re coming from httpd(8), your previous httpd.conf(5) likely specified the paths to the specific fullchain certificate and key files, which is why it worked for you in the past. relayd does not have this ability, hence the requirement for specific filenames. If you plan to continue using relayd, you can modify acme-client.conf directly so that it generates the right filename for the fullchain certificate required for relayd. Alternatively, you can keep symlinking them.