@johnpoz
Thank you again for explaining!
I have a German Telekom VDSL with true dualstack, get a dynamic public IPv4 (and v6) from that ISP.
I get an /56 prefix. The first router is a fritzbox, pfsense is set as exposed host for IPv6 and (as mentioned) gets the GUA for WAN by dhcp from fritzbox. It cuts a /59 net out of the ISP's /56 (fritzbox uses two /64 for LAN and Guest-LAN) as described in this forum a few times by others)...
Since everything intern and (trusting those various IPv6 test sites) extern seems to work just fine (intern working with track interface, slaac, no active dhcpv6 servers) I put the topic to a rest.
Thank you for taking time to elaborate the log entry!