Matt, thanks for your elaborate answer. It turned out I was actually able to attach briefly, but a crash appeared that took me a while to understand. After paying close attention to the logs in the Console, I found out that the following message appeared just before the crash: This copy of libswiftCore.dylib requires an OS version prior to 12.2.0 After reading about it online, I resolved this issue by adding /usr/lib/swift as the first item in my Runtime Search Paths for the tunnel provider target. After adding this item Instruments works as expected again. I'm not at all sure if it's at all related to Instruments at this point, but I hope that future readers find this useful.

While debugging using a HTTPS proxy I often use the user agent to distinguish between requests. This can most certainly be done differently. After writing this post, I realize that I might have misinterpreted the release notes. I now interpret this issue as being fixed since beta 4. Thanks for following up!

I reported a bug with ticket no 8889361.

I have the same question actually. I've posted about similar changes in iOS 14 before (https://developer.apple.com/forums/thread/657824). Is there any chance this can get reinstated?

No, not exactly. In that case last year, manually set UA’s would not be respected in CONNECT requests for any NSURLRequests. The behavior that I currently see is that no user agent whatsoever is passed for CONNECT requests. This feels quite arbitrary to me, as all other requests (GET/POST/…) do function as expected.

I can confirm that this bug is still present in iOS15 beta 2.

Update: this bug still exists in iOS 15 beta 3. Is there any movement in the bug reports mentioned in this thread?