Found this in the release notes for beta 6 under known issues: https://developer.apple.com/documentation/xcode-release-notes/xcode-15-release-notes
@Model classes with initial values for stored properties result in an error that self._$backingData is used before being initialized. (113572344)
and
visionOS projects that use the @Observable property wrapper will fail to build in Xcode 15 beta 3. (111494849)
Seems related...
davidweiss
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This is with Xcode Version 15.0 beta 6 (15A5219j)