I’m not surprised that doesn’t work, because we use connman as our network manager, so it will just override those manual settings.
To disable ipv6 on Ethernet you would use connmanctl:
osmc@rpi2:~$ connmanctl
connmanctl> services
*Ac Wired ethernet_b827ebd04564_cable
connmanctl> config ethernet_b827ebd04564_cable --ipv6 off
connmanctl> quit
This change is persistent. Configuring the network interfaces from the command line is all done via connmanctl - type help while you’re in there and you’ll see there is a lot that can be done.
Tip - rather than typing out the full ethernet string just type e and hit tab…