Hi AndyfF360
The best way is to use the new-ish "Android Enterprise System App" type option when creating a new app. This allows you to add the built-in System apps that you either blocked by using KME or using a Kiosk build. What I did was to install the GET APK app. This allows you to see what the application names are for all the System and Installed apps on your device. for example the Phone app is called com.samsung.android.app.telephonyui.
You use this as the Package Name, when creating the new app. This method allows you to add system apps to your build. I previously used 3rd party apps for Phone, Camera, Calendar etc. Now, I only use the system apps for the standard apps.
The PowerUI issue has been causing me grief since June. I have had an escalated call open with MS since June. They are now (allegedly) sorting this out with Google. Basically the PowerUI no longer has the permission to appear on top of the Managed Home Screen. When you exit Kiosk mode the PowerUI options appear. They thought they had resolved the issue a few weeks back, but, it only worked for the first time the device was switched on after building.
I am chasing them on a daily basis.