While I have not confirmed it as a solution yet, this post appears to cover the issue: https://developer.apple.com/forums/thread/662688

I have a workaround, but I'm not 100% sure I understand why the Shape properties are nonisolated so I'm not sure if this actually safe or not. struct SimpleShape: Shape { var width: Double nonisolated var animatableData: Double { get { MainActor.assumeIsolated { width } } set { MainActor.assumeIsolated { width = newValue } } } func path(in rect: CGRect) -> Path { return Path() } }