It seems like this is only a bug in the visual representation in the WebUI, as multiple DNS servers get pushed when using multiple IPs in one option (e.g. 6,192.168.1.40,22.214.171.124,126.96.36.199):
However, it seems like the DNS servers not shown in the WebUI need to be removed using the command line. To change an already existing DHCP entry, use these commands:
uci set dhcp.lan.dhcp_option='6,192.168.1.40,188.8.131.52,184.108.40.206'
uci commit dhcp
Then, either renew the DHCP lease on your devices or simply reboot the router. Command /etc/init.d/network restart should also work.
As for using separate lines for each DHCP option, I don't believe this should work but I will ask the RnD team.
This bug will be reported to our RnD, thank you for letting us know!