I have opened a Syncthing node behind a NAT to the internet following the docs. As I have UPnP disabled, I have opened 22000/TCP and 21027/UDP ports on my router pointing to that device. However I also see that other nodes are connected to that node through quic://external_ip:22000. That would be 22000/UDP (which is not open in my router config) instead of TCP since quic uses UDP. Is Syncthing punching a hole through the NAT by using RELATED packets?
nmap reports the port as open|filtered, which means that the port is giving back no response.