After a bunch of debugging in the llvm-profdata source code to understand how .profraw Version 8 files work, and then doing some hex editing of my corrupted output .profraw file in DerivedData to remove the corrupted section of the file, I think I have determined which framework we include that is breaking my project: MBProgressHUD. After all of that, I was able to see a simple way of determining the included frameworks that will corrupt the ProfData: Go to the Carthage/Build folder (or wherever your included .xcframework folders are) and look at the modified date of each framework's Info.plist. Any really old ones are likely to break, and the only one I have in this project before 2023 is the one for MBProgressHUD (which is from 2021, which is definitely way before XCode 14.3).

I'm seeing the same issue in all of my projects on xcode 15.0.1. For folks who got it fixed, how did you figure out which upstream dependency was breaking things?

I think this is due to this other xcode 14.3+ issue and nothing wrong with Slather or Fastlane: https://developer.apple.com/forums/thread/734611