I have attached all the investigation details and the crashlogs in the reply here" Thanks for looking into this.

@meaton Can I check why would you say in your last sentence that H3 "may" be used on subsequent attempts to the server? Are there other factors that determine this as well? Asking because as per our tests we are only able to get H3 working by setting assumesHTTP3Capable and not via the Alt-Srv header.

@meaton Noted. Thanks for the headsup. We are trying to debug why newer connections created do not use HTTP3. Is there any diagnostics that we can look at to identify potential issues why our app does not use HTTP3 despite the server advertising the necessary Alt-Srv?

Thanks for the info! In our tests, we are noticing that despite expiring the connection and creating a new one by putting the device on and off aeroplane mode, the newly created connection is still using HTTP2. This is post the previous connection receiving both of the following. IN HTTPS alpn="h3,h2" Alt-Svc: h3=":443"; ma=2592000 Do you have a suggestion on how we could debug this? We are preferring to use the above Service Discovery mechanism instead of assumesHTTP3Capable.

Facing the same since 15.3. Happening in 15.4 as well. Simulator just hangs after this showing this error.