On one Linux Client, ip -6 neighbor lists the Proxmox VE Host(s) as Router, when they clearly are not:
2XXX:XXXX:XXXX:1::A:B dev eno1 lladdr 11:22:33:44:55:66 router REACHABLE
Setting sysctl -w net.ipv6.conf.vmbr0.forwarding=0 on the Proxmox VE Host fixed that, but I wonder if that also caused...