I discovered the above mentioned bug: the "routing on the host" quagga does not announce ipv6 prefixes configured on dummy0 or lo, if a route-map is configured for redistribution under ospf6.
If I configure "redistribute connected" instead, the prefixes get announced as intended.
If I configure the prefixes on anonther interface like bond0.123, it gets announced as intended.
The stock Ubuntu quagga does not have this bug.
Please let me know, if you need any further information. Any workaround or other hint would really be appreciated.