Problem running AR scene in Unity 2020 - No active XR Subsystem errors

I successfully built a very simple AR scene in Unity 2019 and built it to Android. However trying to replicate my project on Unity 2020.3.10f or 2020.3.15f is causing me issues (below).
My basic setup is:

• Open a new scene

• File > Build Settings > Android > add scene and switch platform

• Install latest verified AR Foundation (4.1.7) & ARCore XR Plugin (4.1.7)

• Project Settings > XR Plugin Management > Android> Check "Initialize XR on StartUp" & "ARCore"

• Delete Main Camera

• Add AR Session & AR Session Origin to the scene

Normally now in 2019, if hit Play it would work. But in 2020, I hit play, the viewport is black, and I am getting the following errors:

 No active UnityEngine.XR.ARSubsystems.XRSessionSubsystem is available. Please ensure that a valid loader configuration exists in the XR project settings.
 
 UnityEngine.XR.ARFoundation.ARSession:OnEnable () (at Library/PackageCache/com.unity.xr.arfoundation@4.1.7/Runtime/AR/ARSession.cs:341)
 
 No active UnityEngine.XR.ARSubsystems.XRCameraSubsystem is available. Please ensure that a valid loader configuration exists in the XR project settings.
 
 UnityEngine.XR.ARFoundation.SubsystemLifecycleManager`3<UnityEngine.XR.ARSubsystems.XRCameraSubsystem, UnityEngine.XR.ARSubsystems.XRCameraSubsystemDescriptor, UnityEngine.XR.ARSubsystems.XRCameraSubsystem/Provider>:OnEnable () (at Library/PackageCache/com.unity.xr.arfoundation@4.1.7/Runtime/AR/SubsystemLifecycleManager.cs:87)

 No active UnityEngine.XR.XRInputSubsystem is available. Please ensure that a valid loader configuration exists in the XR project settings.
 
 UnityEngine.XR.ARFoundation.ARInputManager:OnEnable () (at Library/PackageCache/com.unity.xr.arfoundation@4.1.7/Runtime/AR/ARInputManager.cs:24)
 

I've searched far and wide, and all the forum posts I can find just say to make sure to check the boxes in XR Plugin Management, but this is done and it makes no difference.

I can replicate the same error on two separate machines.

Thanks for the advice.