Actor-Isolated Class with Different Actor Isolation

I've got the new beta of Xcode 13 (beta 4 + 5), and now I get this build error:

Main actor-isolated class 'My Class' has different actor isolation from nonisolated superclass 'My Super Class'

Not sure how to fix this one, I'm not using any actors.

Does anyone know what this means?

I get the error on this line.

Related Apple Forums post.

Edit:

I found out that a simple protocol extension, in the app, of a class in the package can cause this:

protocol Test {
    var test: String? { get }
}

extension ImagePIX: Test {
    var test: String? { nil }
}

Prefixing the protocol with @MainActor solves the bug:

@MainActor protocol Test { ... }

Edit:

Fixed in Xcode 13 RC