What I could figure out:
You configured a vpn base on Tailscale.
Initially the PVE cluster is working.
But if some IPs change, not anynmore.
Questions:
The Tailscale vpn is established by separate devices?
You mentioned subnet routers, so after the Tailscale vpn there is a router and then the PVE node?
If the Tailscale vpn has some renegotiation taking place, maybe due to new ip address assignment on public side, PVE cluster nodes can't communicate with each other anymore?
Aside of these question, please evaluate the documention reagarding network requirements in a Cluster:
https://pve.proxmox.com/wiki/Cluster_Manager#_cluster_network
As long as you can't guarantee the stated conditions, it will not work reliably anyway and all troubleshooting is for nothing.