Hello there,
We might need to dig deep into your configuration to see if everything is set right.
Try adding Table = 123 under [Interface] in Server1's WireGuard config. and then running the command ip rule add iif wg0 table 123.
You need to run netfilter-persistent save even after running ufw commands for the firewall rules to be persistent across reboots even if ufw status says they are in place. If for some reason you reboot before saving, delete the UFW rules and add them again and then run netfilter-persistent save.
Hope this resolves your Query !!
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