Yes you are correct. The current implementation of connman doesn't allow you to set up stateless address auto configuration (SLAAC). The 'auto' setting which should enable SLAAC or DHCPv6 only gives you a link local address. Same for the settings regarding ipv6 in the interface. They don't correctly set a fixed ipv6 address.
The only way I found to get it working is to manually set a fixed ipv6 address with connmanctl in terminal. My provider sometimes changes the WAN prefix and then I have to manually change it on my LE box. So yes, this is far from ideal, but it only happens like ones in 6 months. I can live with that.
Basically the ISP's should not have to change it because there are more then enough ipv6 addresses for everybody, so keeping it fixed all the time would save us a lot of hassle, but it probably is because they use DHCP-PD to assign ipv6 prefixes to their customers and you just get auto assigned a free one out of the pool.
My guess is that SLAAC doesnt work correctly because of the version of conmann used in LE. I'm not sure what version LE uses, but there were bugs with ipv6 and SLAAC in older connman versions. I haven't looked into it any further since I use port forwards on my LE box, so I really need a fixed IP instead of a SLAAC config.