Thanks! Just to confirm, adding NSAppTransportSecurity group then add NSAllowsArbitraryLoads=YES key to property list helped!

After wrapping the whole thing in one navigationStack, have you encountered any issues presenting a ActionSheet or .sheet with a view on the navigation stack? In my case, when the ActionSheet was dismissed, its parent view (presenting view) is also dismissed (i.e. pop out the navigation stack). (See my question here: https://developer.apple.com/forums/thread/745733 ) Don't know if it only happens to iOS 15, which I have to support at the moment.

Thanks for your suggestion of testing iOS 16. Long story short, I was able to find a real device with iOS 16.2 and the same code works. So it seems likely a bug in iOS 15, namely a background URLSession does not try http/3 first when assumesHTTP3Capable = true. (I had to debug with iOS 15 as I need to support it.) . Is there any chance for such bugs get fixed in iOS 15 future releases?

It seems that App Transport Security only allows specifying domain names for exceptions, while in my case the server only has IP address(es), without domain names. In addition, the server IP address is not static, it's learned via discovery. Thanks for the pointer though.

Is this required only for Swift apps? I have another (older) version of the same app that is written in Objective-C, and XCode allows me to specify different versions of the framework for the different platforms.

My problem is that for some reason Xcode does not allow me to "added constraints for the image size". Please see the screen shot I am attaching in the next post.

"other devices" include Apple devices and non-Apple devices.