@eskimo Hi thank you for your reply.
Potentially, I have not tried to replicate in a test app due to the setup time to replicate a similar environment. Since TCP and UDP were working as expected I was hoping to avoid creating a test app.
If that seems the most logical solution then I'm willing to try it out.
I've currently got my ALPN values as NWProtocolQUIC.Options(alpn: ["h3", "h2", "h1", "h3-29"]) and have tried using the decoded sni to create the NWConnection rather than raw IP but that hasn't changed the outcome.