It looks like it's an issue with the extension not activating. I removed the extension target, which also removed the embedded .appex from my main app target, and I get the exact same result. It seems like it can't find the extension.

Through beta 5 now, same result.

Still going back and forth on this issue. This does work perfectly fine as long as Private Relay isn't enabled. For now, our alternate plan if we can't get this resolved is to tell our customers to turn off Private Relay. We'll add it to our help articles, customer service troubleshooting steps, chatbot, etc.

One of my end users is seeing this issue (seen via sysdiagnose logs from that user). It's causing things to break only for that user. Two things I noticed about this user: It only seems to happen for them connecting to a school network where they have to use their login credentials to gain access to the network. Even after successfully connecting to the school wifi network, the NWPath still has the preferred (first) network as pdp_ip0. In my testing after connecting to WiFi, the preferred network becomes en0. This user is on AT&T. I don't have a test device with AT&T LTE to test this. Are either of these true for your case?

Just re-tested as of iOS 15.3.1 and this issue doesn't appear to be there anymore for iOS.