I'm having exactly the same issue with a Debian 10 LXC running the Mullvad client when migrating from PVE 6.4-13 to 7.1-10. The Mullvad client is using wireguard; I've tried running wireguard directly and get similar results.
arch: amd64
cores: 1
hostname: vpn-gateway
memory: 128
nameserver...
