I have same observation with BETA 2 or Public beta. The userAgents appear in random case. The results are not consistent. It seems more like a defect in iOS 15. Also, the behavior is same with or without iCloud Private Relay ON/OFF.

I have the same behavior while using on Device VPN (NEPacketTunnelProvider). No User Agents (at least 95% of the times). Randomly, I see few user Agents reported on some traffic. We have a webpage that renders different functionality / content based on the operating system. We user UserAgent to match for OS @eskimo

Any updates to this original bug report? Mine (Bug # FB9329234) was resolved stating - working as designed.