I had Python 3 in my PATH before the 1903 update, and the 1903 update still added those two "rogue" python files in my WindowsApps folder. Even after removing that folder from my user PATH (it wasn't in my system PATH), and after rebooting, typing "python" in PowerShell opened the store. The WindowsApps folder was at the end of my PATH variable anyways, so it should not have overridden the working version, but it did. Get-Command python even returned the python.exe in WindowsApps.