If nmcli refuses to change ipv6.method because of leftover settings, the fastest fix is to recreate the connection with IPv6 disabled.
# Show current connections
nmcli con show
# Delete old connection
nmcli con delete "System eth0"
# Recreate with IPv4 only (replace IP/GW with yours)
nmcli con add con-name "eth0" ifname eth0 type ethernet \
ipv4.addresses 192.168.1.100/24 ipv4.gateway 192.168.1.1 \
ipv4.method manual ipv6.method disable
# Bring it up
nmcli con up "eth0"
# Verify
nmcli con show "eth0" | grep ipv6.method
ip a | grep inet6
✅ Result: ipv6.method: disable and no IPv6 addresses.