Linux VMs in KVM running through a bridge interface don't get any IPv6 despite seeing router advertisement with tcpdump

I've been fighting this for a while now and am at a loss on how to debug this further. Since today I noticed ALL Linux VMs on my Unraid server running via KVM through a bridge interface don't get any IPv6 address except their fe80 anymore.

This worked before with both SLAAC and DHCPv6, but something apparently changed and I can't figure out what.

I can see the router solicitation of the VM in the VM on enp1s0, on the Unraid host br0 bridge interface, but not on the OpenWRT router. I can see router advertisements on the VM on enp1s0, but it looks like Linux is ignoring them.

Router advertisement captured in the VM:

# tcpdump -v -i enp1s0 icmp6
tcpdump: listening on enp1s0, link-type EN10MB (Ethernet), capture size 262144 bytes
21:23:35.840097 IP6 (flowlabel 0xfeb59, hlim 255, next-header ICMPv6 (58) payload length: 192) fe80::dea6:32ff:fe1e:6d61 > ip6-allnodes: [icmp6 sum ok] ICMP6, router advertisement, length 192
	hop limit 64, Flags [managed, other stateful], pref medium, router lifetime 1800s, reachable time 0ms, retrans timer 0ms
	 source link-address option (1), length 8 (1): dc:a6:32:1e:6d:61
	 mtu option (5), length 8 (1):  1492
	 prefix info option (3), length 32 (4):, Flags [onlink, auto], valid time 13573s, pref. time 973s
	 prefix info option (3), length 32 (4): fd3b:2815:be50:2::/64, Flags [onlink, auto], valid time infinity, pref. time infinity
	 route info option (24), length 24 (3):, pref=medium, lifetime=1800s
	 route info option (24), length 24 (3):  fd3b:2815:be50::/48, pref=medium, lifetime=1800s
	 rdnss option (25), length 40 (5):  lifetime 1800s, addr: addr:
	 advertisement interval option (7), length 8 (1):  600000ms

The router soliciation as seen on br0 on the Unraid host:

21:19:39.861675 IP6 fe80::dea6:32ff:fe1e:6d61 > ICMP6, router advertisement, length 176

Since I can't see the router solicitation on the OpenWRT router, I assume something is wrong with the bridge interface on Unraid. But everything LOOKS fine. I'm at a complete loss and would be grateful for every suggestion on how to debug this further.

Check any updates in ip6table rules in VMs or host are dropping any traffic ?