I tested and found this error appears on macOS 15.0.1 (24A348)

OK. I'll answer each question: I'm using NEAppProxyProvider combined with NEPacketTunnelProvider and NEDNSProxyProvider. However, I have just encountered the issue with NEAppProxyProvider. All of them are packaged as a sysex. I added some logs to debug and found that the function startProxy was called. Setting setTunnelNetworkSettings also didn't return any errors. Therefore, I think it had started successfully but I didn't see the function handleNewFlow was called. As a result, I see no flows. My app worked stably on older versions but from Sequoia, I have encountered this issue when restarting NEAppProxyProvider. It's an intermittent issue.