Issue with Recursive and back-reference to parent

[fabstu]

Describe the bug
I tried following the HigherOrderReducers-Recursion Use Case, but my app crashes and I don't know why. The difference between the Use Case and my example is, that my recursive state has a back-reference to the parent object, which required the recursive object to be a class instead of a struct.

When accessing attributes on the back-reference .parent in the reducer, the app crashes:

Simultaneous accesses to 0x101955c88, but modification requires exclusive access.
Previous access (a modification) started at libswiftCore.dylib`ReferenceWritableKeyPath._projectMutableAddress(from:) + 1413 (0x7ff81a208575).
Current access (a read) started at:
...

The position where the app crashed is marked below.

Zip:
RecursiveActions.zip

Code:

import Combine
import ComposableArchitecture
import SwiftUI
import CasePaths

public enum IndentDirection: Equatable {
    case left
    case right
}

public final class ItemState: Identifiable, Equatable {
    public let id: UUID
    public var parent: ItemState?
    public var children: IdentifiedArrayOf<ItemState> = []
    public var content: String
    
    public init(
        id: UUID = UUID(),
        parent: ItemState? = nil,
        children: IdentifiedArrayOf<ItemState> = [],
        content: String
    ) {
        self.id = id
        self.parent = parent
        self.children = children
        self.content = content
        
        // Set parent hierarchy correctly to make manual initialization less clunky.
        for child in children {
            child.parent = self
        }
    }
    
    public static func == (lhs: ItemState, rhs: ItemState) -> Bool {
        return lhs.id == rhs.id &&
            lhs.children == rhs.children &&
            lhs.content == rhs.content
    }
}

public indirect enum ItemAction: Equatable {
    case child(id: UUID, action: ItemAction)
    case editText(newText: String)
    case indent(direction: IndentDirection)
}

public struct AppEnvironment {
    public init(mainQueue: AnySchedulerOf<DispatchQueue> = .main) {
        self.mainQueue = mainQueue
    }
    
    var mainQueue: AnySchedulerOf<DispatchQueue>
}

extension Reducer {
    static func recurse(
        _ reducer: @escaping (Reducer, inout State, Action, Environment) -> Effect<Action, Never>
    ) -> Reducer {
        
        var `self`: Reducer!
        self = Reducer { state, action, environment in
            reducer(self, &state, action, environment)
        }
        return self
    }
}

public let nestedReducer = Reducer<
    ItemState, ItemAction, AppEnvironment
>.recurse { reducer, state, action, environment in
    switch action {
    case .child:
        return reducer.forEach(
            state: \ItemState.children,
            action: /ItemAction.child(id:action:),
            environment: { $0 }
        )
        .run(&state, action, environment)
        
    case let .editText(newText):
        state.content = newText
        return .none
        
    case let .indent(direction):
        //FIXME: Crashes on access to .children.
        state.parent?.children
        return .none
    }
}
.debugActions()

Test case:

import XCTest
import ComposableArchitecture
import SwiftUI
import RecursiveActions

class TestItemState: XCTestCase {
    func testActionsIndent() throws {
        let parentUID = UUID()
        let uid1 = UUID()
        let uid2 = UUID()
        let text1 = "Hello"
        let text2 = "World"
        
        let store = TestStore(
            initialState: ItemState(id: parentUID, children: [
                .init(id: uid1, children: [], content: text1),
                .init(id: uid2, children: [], content: text2),
            ], content: ""),
            reducer: nestedReducer,
            environment: AppEnvironment()
        )
        
        store.send(.child(id: uid2, action: .indent(direction: .right))) // Fails.
        store.send(.indent(direction: .right)) // Works.
    }
}

Expected behavior
Do not crash on accessing the parent reference.

Environment

• swift-composable-architecture version 0.33.1
• Xcode Version 13.3 beta 2 (13E5095k)
• Swift 5.4
• OS: MacOS 12.2.1 (21D62)

Additional context
I'm not knowledgeable enough to understand this crash, is this a limitation with TCA? How else would one model such a scenario, where access to the parent is necessary? Thanks in advance for any ideas.