I have done several successful migrations in the past that usually involve either adding an item to the @Model or renaming an item.

I recently added 2 items. I duplicated the last Schema, added the 2 items, bumped up the Schema version, did the migration (all of these steps having been done multiple times before with success), and now the app crashes with _explanation: (String?) = nil. All items have been given default values.

I've literally done nothing different than I've done before. I am completely confused. I added an item in the last Schema/Migration and all worked. But adding an item with next Schema/Migration and it fails.

But if I delete the app on the simulator (or device) and run it fresh with this SchemaV15, it does not crash.

My Schema:

enum SchemaV15: VersionedSchema {
    static var versionIdentifier: Schema.Version {
        return Schema.Version(15,0,0)
    }
    static var models: [any PersistentModel.Type] {
        [
            Note.self,
            PBStatsDetail.self,
            MyNotes.self
        ]
    }

    @Model
    class PBStatsDetail {
        var id: UUID
        var timestamp: Date
        var totalSU: Int64 = 0

        init(
            id: UUID,
            timestamp: Date,
            var totalSU: Int64 = 0
        ) {
            self.id = id
            self.timestamp = timestamp
            self.totalSU = totalSU

my main struct app:

@main
struct Pickleball_DrillsApp: App {
    
    typealias MyNotes = SchemaV15.MyNotes
    typealias Note = SchemaV15.Note
    typealias PBStatsDetail = SchemaV15.PBStatsDetail
    
    let container: ModelContainer
    
    init() {
        do {
            container = try ModelContainer(
                for:
                MyNotes.self,
                Note.self,
                PBStatsDetail.self,
                migrationPlan: DBMigrationPlan.self
            )
        }
        catch {
            fatalError("Failed \(error)")
        }
    }//init
    
    
    var body: some Scene {
        WindowGroup {
            ContentView()
                .modelContainer(container)
                .environmentObject(Favorites())
        }
    }
}

enum DBMigrationPlan: SchemaMigrationPlan {
    
    static var schemas: [any VersionedSchema.Type] {
        [
            ......
            SchemaV14.self,
            SchemaV15.self
        ]
    }
    
    static var stages: [MigrationStage] {
        [
            ......
            migrateV13toV14,
            migrateV14toV15
        ]
    }
    
    static let migrateV14toV15 = MigrationStage.custom(
        fromVersion: SchemaV14.self,
        toVersion: SchemaV15.self,
        willMigrate: nil,
        didMigrate: nil
    )