The accept_ra_rt_info_max_plen sysctl defaults to 0

For router advertisements that includes route information. Any route more specific than accept ::/0 will be rejected by the kernel by default.

This value can be configured as follows for a prefix of ::/96 for example:

sysctl net.ipv6.conf.<interface_name>.accept_ra_rt_info_max_plen=96

To make the setting persistent add the ...