IPv6 containers sometimes not reachable

General
1

I run docker on Debian 11 with IPv6 enabled (“fixed-cidr-v6”: “2001:db8:1::/64”). I use docker compose to assign certain containers (e.g. traefik) a globally reachable IPv6 via the network section:

networks:
  public_ipv6:
    name: public_ipv6
    enable_ipv6: true
    driver: bridge
    ipam:
      driver: default
      config:
        - subnet: 2a00:abcd:abcd:abcd:1::/80

With ndppd I automated ndp:

proxy enp1s0 {
  rule 2a00:abcd:abcd:abcd:1::/80 {
    auto
   }
 }

And I applied the following config:

sysctl net.ipv6.conf.enp1s0.accept_ra=2
sysctl net.ipv6.conf.default.forwarding=1
sysctl net.ipv6.conf.all.forwarding=1
sysctl net.ipv6.conf.enp1s0.proxy_ndp=1
sysctl -p /etc/sysctl.conf

Most of the time my container with the ip http://[2a00:abcd:abcd:abcd:1::2]/ is reachable (by ping and via browser) through the internet, but quite some times it is not.
The firewall is set correctly. I also don’t have this problem when using the hosts standard IPv6 (of the enp1s0 adapter).

I ran the ip monitor and I really get a lot of events per minute for this IP:

ip -6 monitor |grep abcd
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 STALE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 REACHABLE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 STALE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 PROBE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 REACHABLE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 STALE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 REACHABLE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 STALE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 REACHABLE
2a00:abcd:abcd:abcd:1:2 dev br-randomstuff lladdr 02:42:ac:15:00:02 STALE

This is what it looks like from outside of my network:

root@localhost:~# ping6 whoamiv6.mydomain.de
PING whoamiv6.mydomain.de(2a00:abcd:abcd:abcd:1::2 (2a00:abcd:abcd:abcd:1::2)) 56 data bytes
64 bytes from 2a00:abcd:abcd:abcd:1::3 (2a00:abcd:abcd:abcd:1::2): icmp_seq=1 ttl=56 time=14.0 ms
From 2a00:abcd:1000:43::1423 (2a00:abcd:1000:43::1423) icmp_seq=2 Destination unreachable: Address unreachable

Is there anything I’m missing? I’m happy to give more info. Right now I’m not sure where to look.
Any help is appreciated!