Sendable conforming with @Observable macros

Using the class below:

final class AppState: Sendable {
	enum Stage: Sendable {
		case authorizationRequired
		case authorized
	static var shared = AppState()
	var currentStage: AppState.Stage
	// Warning: Stored property '_currentStage' of 'Sendable'-conforming class 'AppState' is mutable

	// … other code

I get the following warning:

Applying the @MainActor doesn't resolve the issue. Documentation said:

Contain only stored properties that are immutable and sendable

I do understand what the warning says, but I need the mutability.
I have read a similar topic, but I don't understand what I need to do ?