In Xcode 26.5, 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.