Confirming this issue on macOS 27, Xcode 27, UE 5.8.2, using Modern Xcode IOS build.
Build fails at the “Touch UBT generated tiles” scheme pre-action script with zero output. No error, no log line from the script itself, just “BUILD FAILED” and “Failed to finalize the .app with Xcode”.
Granting disk access to dotnet or other Unreal tools didn’t help. Also tried disabling UBA, disabling Spotlight indexing, and running the build scripts with elevated permissions - no change.
Building from Xcode project directly works fine. “Touch UBT generated tiles” pre-action runs without issue there. It fails when building via terminal / Editor. But building through Xcode doesn’t cook content, so you first need at least one successful terminal/Editor build to get cooked content before Xcode builds are actually usable.
The only workaround I found: fully wipe Binaries, Saved and Intermediate (project + plugins), Xcode/DerivedData and delete the generated .app before every build. A clean build from terminal lets the pre-action script run. Just deleting the .app isn’t enough. Even with a full wipe it’s not 100% reliable, but it’s the only thing that consistently helps.
And there’s no way to run Xcode 26 on macOS 27…
UPD: for iOS SDK 27 you also need bUseSceneBasedLifecycle enabled, otherwise you get a crash. And to actually use it you need to rebuild the ApplicationCore and Launch Engine modules with that flag - without the rebuild you just get a black screen instead of a crash.