Basically, take just the Xcode 26 AR App template, where we put the ContentView as the detail end of a NavigationStack.
Opening app, the app uses < 20MB of memory. Tapping on Open AR the memory usage goes up to ~700MB for the AR Scene. Tapping back, the memory stays up at ~700MB.
Checking with Debug memory graph I can still see all the RealityKit classes in the memory, like ARView, ARRenderView, ARSessionManager.
Here's the sample app to illustrate the issue.
PS: To keep memory pressure on the system low, there should be a way of freeing all the memory the AR uses for apps that only occasionally show AR scenes.