Same boat. This change breaks our SDK and just seems odd. Now that Xcode 16.2 has released, using a Task for this seems inappropriate so I'd +1 the MainActor.assumeIsolated but honestly what nonsense and it's frustrating to not know if this change is purposeful. Why are no devs commenting on this; incredibly frustrating.
I like Swift Concurrency but I reckon the devs might be at the point of 'can we make SwiftUI truly concurrent for performance gains' and finding that realistically the answer is no so they need to @MainActor it all. I'm at the same pain point and the biggest and obvious issue is nonisolated contexts that need to access isolated contexts need to be made async so really I need to either @MainActor or async it all.