iPad application transitions to compact size upon entering background. Why?

I have a universal iPad / iPhone application which presents different interface for .regular and .compact horizontal size classes. The architecture of the app is similar to this refined example:

import SwiftUI

struct CompactView: View {
	var body: some View {
		Text("CompactView")
	}
}

struct ContentView: View {
	@Environment(\.horizontalSizeClass) var horizontalSizeClass
	
    var body: some View {
		Group {
			if horizontalSizeClass == .regular {
				NavigationSplitView {
					Text("Sidebar")
				} detail: {
					Text("Detail")
				}
			} else {
				CompactView()
					.onAppear {
						print("Why am I here?")
					}
			}
		}
		.onChange(of: horizontalSizeClass) { newValue in
			print("Size class: \(newValue!), app state: \(UIApplication.shared.applicationState)")
		}
    }
}

Recently I've discovered a strange behaviour: when user presses home button on iPad, application interface changes its horizontalSizeClass from .regular to .compact and back to .regular for a moment when entering background state. And it's sufficient to lose all transient state including presented modals with their state.

The console output for this example in described case is:

Size class: compact, app state: UIApplicationState(rawValue: 2)
Why am I here?
Size class: regular, app state: UIApplicationState(rawValue: 2)

Could anybody explain why this is happening or give any relevant reference?

Yes, I'm aware of the split mode and multitasking and that iPad interface transitions to .compact in those cases. But my question relates to only this one specific case: is this an expected behaviour (and if yes, then why)? Or is it an SDK bug? Thanks.