Difficult to diagnose without an Xcode project. Are you using Xcode’s Archive > Distribute toolchain for signing and notarizing, or doing it manually? Make sure everything you bundle only has x64 binaries – you might need to use lipo from the command line to strip 32-bit sections (or rebuild them from source).
Hey, I’ve found the problem, well at least for know. I’ve removed any sandboxing and that has done the trick. I’ll not be distributing thru the App Store so this is not needed. It would be nice to sort the sandboxing at some point tho’.