The prompt is either being generated by some access to an address on the local subnet directly (which could be some socket, etc), or by something in your app using Bonjour. Most apps that trigger the prompt do so because of browsing Bonjour.
To see what is causing a prompt, you can look at the logs on your device in Console.
Enable Info and Debug level logs in the Action menu. Select your iOS device.
Filter for process:mDNSResponder and message:"local network". This will show you any service that is being browsed that hits a restriction, and the prompting state.
Note that if you don't add that service type into your Info.plist when you compile for iOS 14, the operation will be silently blocked.
If you don't see any relevant logs from mDNSResponder, it's more likely that this is a direct network access rather than a browse.
