What beta are you talking about? I don't see a new macOS 26 or Xcode 26 beta out. They haven't been updated since June 23 / 24.

It is not working for me either. I only downloaded it and tried it under macOS 26 Beta 2 and Xcode 26 Beta 2. The guardrail parameter did not help.

I’m having the same problem. I have a 2TB M1 iPad Pro. It worked with my monitor in various beta builds, but the official build doesn’t work. It just mirrors the screen now and I can’t use stage manager.