Microsoft says that it is working on a fix but, for now, has provided a couple of workarounds to deal with the issue. First, Microsoft says that restarting the Shell Infrastructure host (SIHost.exe) service will help restore the missing Immersive Shell packages. This can be done with the following commands:

Add-AppxPackage -Register -Path 'C:\Windows\SystemApps\MicrosoftWindows.Client.CBS_cw5n1h2txyewy\appxmanifest.xml' -DisableDevelopmentMode  
Add-AppxPackage -Register -Path 'C:\Windows\SystemApps\Microsoft.UI.Xaml.CBS_8wekyb3d8bbwe\appxmanifest.xml' -DisableDevelopmentMode  
Add-AppxPackage -Register -Path 'C:\Windows\SystemApps\MicrosoftWindows.Client.Core_cw5n1h2txyewy\appxmanifest.xml' -DisableDevelopmentMode  

Second, a PowerShell logon script has been shared that essentially blocks Explorer from launching prematurely until the required packages are fully provisioned. The batch script for that is given below:

@echo off  
REM Register MicrosoftWindows.Client.CBS  
powershell.exe -ExecutionPolicy Bypass -Command "Add-AppxPackage -Register -Path 'C:\Windows\SystemApps\MicrosoftWindows.Client.CBS_cw5n1h2txyewy\appxmanifest.xml' -DisableDevelopmentMode"
REM Register Microsoft.UI.Xaml.CBS  
powershell.exe -ExecutionPolicy Bypass -Command "Add-AppxPackage -Register -Path 'C:\Windows\SystemApps\Microsoft.UI.Xaml.CBS_8wekyb3d8bbwe\appxmanifest.xml' -DisableDevelopmentMode"  
REM Register MicrosoftWindows.Client.Core  
powershell.exe -ExecutionPolicy Bypass -Command "Add-AppxPackage -Register -Path 'C:\Windows\SystemApps\MicrosoftWindows.Client.Core_cw5n1h2txyewy\appxmanifest.xml' -DisableDevelopmentMode"  

I swear to god, if I hear “Windows just works” one more goddamn time…