Looking around on the internet for weeks now. For quite some hours searching your forum I cannot find a hint for my issue.
On my RB750GR3 I have installed 7.1.5 and reset to standard configuration.Then added the following:
Renamed the standard bridge on 192.168.88.0/24 to bridge-prive.
Added a guest bridge on 192.168.10.0/24 as bridge-gast.
Configured CAPsMAN for hAP AC3 and Hap Mini. Works perfect! IPv4 works very good on Private as well as on Guest Network.
When I login on the private network (bridge-prive so on subnet 88) and I test my setup with http://test-ipv6.com/ or https://ipv6-test.com/ I have 100% score for IPv4 and IPv6 connectivity.

When I login on the guest network (bridge-gast so on subnet 10) and test I find that it is not IPv6 at all. It looks like something needs to be added to the IPv6 to get the Guest subnet operational on IPv6.

In the IPv6 configuration I see in the /IPv6 Address List where I find for bridge-prive an address in the pool with Advertise=yes and EUI64=yes and an address like fe80::de2c:6eff:fe56:8f52/64. For the bridge-gast there is only an address fe80:1bff:fee1:6e2c/64 but no other address.

My provider gives me an /56 IPv6 address which leaves me 256 subnets of /64. Somehow i believe that it must be possible to use one of those subnets for my guest network, but how? If I try to at an extra address in the pool it says its exhausted.

It would be nice to know what extra to do to get IPv6 working on my guest network.

It should work exactly the same way as it worked with first address (= you need to add it). If not, then check /ipv6/pool/print (prefix should be xxxx:xxxx:xxxx:xx00::/56 and prefix length should be 64) and /ipv6/pool/used/print.

That helped a lot. So now I have 256 subnets with a size of /64, super. IPv6 still does not work on the guest network. Looking for further differences between private and guest Network I arrive at interface lists. That guest net work is not mentioned there. I added the following line to the configuration: So now the two bridges are part of LAN that is used in firewalls.
It works really good now! Thanks for the hint!