Sorry, copy&paste error when I was writing the post. I did use odhcp6c. But DHCP isn't the problem as this is route related.
I captured Router Advertisement. I get an initial RA after RS. But after that, no unsolicited RA's reach my router. So the problems seems to be on the ISP side (again). After I knew what to look for I found similar issues. Seems to be not that uncommon.
I'm waiting to hear back from my ISP. My neighbors use an ISP-provided FritzBox. They don't experience the problem. I'm wondering what a FritzBox does different. Maybe they send RS periodically (violating RFC4861 6.3.7.).