I had the same vexing problem and remembered that often this is due to macOS caching. Since for example it will work for new projects.
Removed all Xcode caches ~/Library/Developer/Xcode/DerivedData/
Rebooted, I also suspect a log out/log in will do.
Rebuild and profiled my app and the source is there, tada.
Topic:
Developer Tools & Services
SubTopic:
Xcode
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