Hello
From reading some topic, some user (including me) suffer from issue that ipv6 adress was not distributed to client (throught odhcpd).
After many headache, the problem was coming from that luci set the value of 'ra_mtu' to 0.
For my setup (xiaomi mi route 4A gigabit) , when this parameters exists in /etc/config/dhcp, odhcpd stop to send router adversis and to reply to sollicitation
Simply delete the line in config restore the ipv6 distribution.
Hope that help