I have no idea. Maybe the tool's broken? Sadly QUIC support is still early in many libs, and so tooling support isn't that great at the moment.
Does your nginx error log have anything to say about issues with the socket?
The listen directive I would expect to work is this one:
listen 443 ssl http2;
listen 443 http3 reuseport; # UDP listener for QUIC+HTTP/3
listen [::]:443 ssl http2;
listen [::]:443 http3 reuseport;