A trivial error appears in the system journal generated by iptables_helper when no tethering is in use:
Apr 11 17:28:38 LibreELEC iptables_helper[258]: grep: technology/wifi: No such file or directory
Apr 11 17:28:38 LibreELEC iptables_helper[258]: grep: technology/ethernet: No such file or directory
Apr 11 17:28:38 LibreELEC iptables_helper[258]: Error 'tether': Invalid argument
Apr 11 17:28:39 LibreELEC iptables_helper[258]: Error 'tether': Invalid argument
I applied a patch to iptables_helper that removed the error - see pastebin
Cheers,
Simon