In Xcode project > Build Settings > Swift Compiler - Concurrency. When we have those settings : Approachable Concurrency - Yes Default Actor Isolation - MainActor A sendable struct without @Actor annotation will be stuck to @MainActor. But if we have a sendable struct, by principle, it should be used across Actors. To remediate the situation, we had to prefix the struct with nonisolated keyword. The setting "Default Actor Isolation - MainActor" should not add @MainActor to Sendables. Problem describe in : FB23264607