Advertise a subnet route with radvd
I have found the problem.
Per default, the Linux kernel does only accept default routes via the router advertisement options in icmpv6.
To fix this, the correct kernel parameter must be set:
net.ipv6.conf.all.accept_ra_rt_info_max_plen = 128
From kernel docs:
accept_ra_rt_info_max_plen - INTEGER Maximum prefix length of Route Information in RA.
Route Information w/ prefix larger than or equal to this variable shall be ignored. Functional default: 0 if accept_ra_rtr_pref is enabled. -1 if accept_ra_rtr_pref is disabled.