Swift UI on iOS 14 not assigning new object to @State property

On iOS 13 I used to use optional @State properties to adapt views. In my case, the presented view would either create a new object (an assignment) if the state that is passed into it is nil, or edit the assignment if an assignment was passed in. This would be done in the action block of a Button and it worked beautifully.

On iOS 14 / Xcode 12 this no longer seems to work. Given the following code which creates a new assignment and passes it into the editor view when the user taps a "New Assignment" button, the value of assignment remains nil. Is anyone else experiencing similar behaviour?

Code Block Swift
struct ContentView: View {
		@Environment(\.managedObjectContext) var context
		@State var assignmentEditorIsPresented = false
		@State var assignment: Assignment? = nil
		var Body: some View {
				[...]
				Button("New Assignment", action: {
						self.assignment = Assignment(context: context)
						self.assignmentEditorIsPresented = true
				})
				.sheet(isPresented: assignmentEditorIsPresented) {
						[...]
				}
		}
}

What's even weirder is that I tried adding a random piece of state, an Int, to this view and modifying it right before the assignment state (between lines 9 and 10) and it didn't change either.

Code Block Swift
Text("\(assignment?.description ?? "")"
	.hidden()