Loss of IPv6 Adress after 6 Minutes

[Doriangrey]

My ISP makes Prefix Delegation (DHCPv6-PD) and delegates /56 Prefixes to its customers.

First I changed from Cable to FTTH (New Modem) and I struggled to keep IPv6 Up and Running, but somehow it works now.
Some Months later I wanted to activate IPv6 for a customer, with the same settings I couldn't get it up and running.

My feeling tells me that my Home IPv6 Link stays just out of luck, Up- and Running.

What i observed:
When I activate IPv6 it works right away after a reload and the Interface gets its own IPv6 Address, Prefix, and IPv6 Prefixed Range. The WAN-Overview shows that I have a /56 prefix delegated with its corresponding Range. All LAN Clients work at that point. Constantly after six Minutes, the IPv6 of the WAN interface stays Up and Pingable from LAN, but all Public IPv6 addresses (example ipv6.google.com) aren't reachable anymore.

OS:
OPNsense 22.1.9_1-amd64
FreeBSD 13.0-STABLE
OpenSSL 1.1.1o 3 May 2022

Settings:
IPv6 DHCP -> Prevent release is ON (because of that, but it does not help. Somehow, the Connection from WAN to my ISP gets Dropped on the IPv6 side.)

Settings right now:
IPv6 Configuration Type -> DHCPv6
Prefix delegation size -> 56
Send IPv6 prefix hint -> OFF

FW Settings:
Allow IPv6 -> ON
ICMPv6 isn't blocked

Hint:
I tried also a Tunable, which was mentioned by some users of the same ISP net.inet6.icmp6.nd6_onlink_ns_rfc4861 = 1, but this didn't help.

.. I leave out the RA part, because the connectivity from LAN to my WAN Interface stays all the time, so IMHO this shouldn't be an issue.

I have a log, but send it only as PM to someone because of security reasons