Sounds like my issue, and found that this problem can occur after you've upgraded your macOS beta. I know this because I had a working app on a previous Xcode beta, and it was only until I updated my macOS beta that this problem occurred. See my issue here:
[(https://developer.apple.com/forums/thread/734970)]
dkk277
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Can you check if your SwiftData properties have default values, and if so, remove them and just set them from init? So if you have a property called var creationDate: Date = Date() change to var creationDate: Date, and in init() set the date to Date(). Let me know the result, I'm curious!