With my project built with Xcode 16.0 (16A242d), I experience this same behavior on iOS 18.0. However, the same build on iOS 18.1 beta 5 is not. (actual devices) So perhaps this is fixed, though I could not find any mention in release notes. Can anybody confirm the same results?

For the last 30 minutes my e-mail box has been bombarded by Xcode Cloud failure messages, repeating over and over again.

Xcode 13 Beta seems to have addressed this issue. Having installed it, I can actually run my UI tests again in Xcode 12 as well.