I'm seeing the same issue in macOS Monterey, Beta 1 (12.0 Beta (21A5248p)), works fine in iOS 15 beta2, but crashes in SwiftUI.OutlineListUpdater in MacOS.

years later, I have hit this issue with macOS Monterey beta 1, even restarting the Mac app it doesn't look to be pulling in the changes, but any changes made are pushed to the iOS side fairly quickly. This is all though the development environment in cloudkit, and running the Mac app via Xcode, so maybe production works better. update: things randomly started working, so my particular issue may have been code related more so than push notifications.

I experimented with this briefly, and if you have the iCloud entitlement with a CloudKit container selected it does automatically sync. However it still has all the same restrictions Core Data has for CloudKit integration such as not supporting attributes like Unique. In the same vein, I have yet to see any sort of Persistent History delegates or observations with SwiftData, such as in my case I have to deduplicate entries because of the lack of unique attributes in CloudKit. So overall it's great API for simple data models, but quickly runs into needing to drop back to Core Data for anything more realistic.