The Logon As function is determined by the StaffProxyWebUrl key in Customization Manager. If this key is changed, it can affect what is seen using the Logon As function.
Signing into a User's Web Account from the Client
Make sure the StaffProxyWebURL Key has a value that points to the TestWeb address like this example: https://illiadaddress.site.edu/ILLiad/TestWeb/